It is true that a lot of Muslims believe this. However, there are others that believe she was the Prophet's (peace be upon him) own Wife.
Taken from http://www.islamonline.net/servlet/Satellite?pagename=IslamOnline-English-Ask_Scholar/FatwaE/FatwaE&cid=1119503543986
The Prophet (peace and blessings be upon him) contracted some of his marriages for sociopolitical reasons. His principal concern was the future of Islam. He was interested in strengthening the Muslims by all bonds. That is why he married the young daughter of Abu Bakr, his First Successor, and the daughter of `Umar, his Second Successor. It was by his marriage to Juwayriyyah that he gained the support for Islam of the whole clan of Bani Al-Mustaliq and their allied tribes. It was through marriage to Safiyyah that he neutralized a great section of the hostile Jews of Arabia. By accepting Mariyah, the Copt from Egypt, as his wife, he formed a political alliance with a king of great magnitude. It was also a gesture of friendship with a neighboring king that Muhammad married Zaynab who was presented to him by the Negus of Abyssinia in whose territory the early Muslims found safe refuge.
Ibn Kathir is quoted to have said:Maria al-Qibtiyya (may Allah be pleased with her) is said to have married the Prophet (peace and blessings of Allah be upon him) and certainly everyone gave her the same title of respect as the Prophet's wives, 'Umm al Muminin' 'Mother of the Believers'.The muqawqis, or King of the Copts, gave the Prophet (S) two wives as a gift. The Prophet (S) married one, who became the mother of the Prophet's son Ibrahim, and he gave the other one to Hassan as a wife.(Haythami said in Majma al-zawaid, "Tabarani in al-Aswat and al-Bazzar related it, and the narrators in al-Bazzar's chain are all scholar's of sound hadith as stated by Hisham Kabbani in Encyclopedia of Islamic Doctrine: Remembrance of Allah and Praising the Prophet, p. 76 Vol. II)
For more evidence that Mariya was indeed the Prophet's wife, visit this sitehttp://www.irfi.org/articles/articles_201_250/honor_of_the_noble_prophet.htm
Umar Ibn Khattab explains that these two women were Hafsa and Ayesha who became disrespectful of the prophet causing him grief until he thought of divorcing all of his wives. Here is the full story.
Bukhari Volume 3, Book 43, Number 648:
Narrated 'Abdullah bin 'Abbas:
I had been eager to ask 'Umar about the two ladies from among the wives of the Prophet regarding whom Allah said (in the Qur'an saying): If you two (wives of the Prophet namely Aisha and Hafsa) turn in repentance to Allah your hearts are indeed so inclined (to oppose what the Prophet likes) (66.4), till performed the Hajj along with 'Umar (and on our way back from Hajj) he went aside (to answer the call of nature) and I also went aside along with him carrying a tumbler of water. When he had answered the call of nature and returned. I poured water on his hands from the tumbler and he performed ablution. I said, "O Chief of the believers! ' Who were the two ladies from among the wives of the Prophet to whom Allah said:
'If you two return in repentance (66.4)? He said, "I am astonished at your question, O Ibn 'Abbas. They were Aisha and Hafsa."
Then 'Umar went on relating the narration and said. "I and an Ansari neighbor of mine from Bani Umaiya bin Zaid who used to live in 'Awali Al-Medina, used to visit the Prophet in turns. He used to go one day, and I another day. When I went I would bring him the news of what had happened that day regarding the instructions and orders and when he went, he used to do the same for me. We, the people of Quraish, used to have authority over women, but when we came to live with the Ansar, we noticed that the Ansari women had the upper hand over their men, so our women started acquiring the habits of the Ansari women. Once I shouted at my wife and she paid me back in my coin and I disliked that she should answer me back. She said, 'Why do you take it ill that I retort upon you? By Allah, the wives of the Prophet retort upon him, and some of them may not speak with him for the whole day till night.' What she said scared me and I said to her, 'Whoever amongst them does so, will be a great loser.' Then I dressed myself and went to Hafsa and asked her, 'Does any of you keep Allah's Apostle angry all the day long till night?' She replied in the affirmative. I said, 'She is a ruined losing person (and will never have success)! Doesn't she fear that Allah may get angry for the anger of Allah's Apostle and thus she will be ruined? Don't ask Allah's Apostle too many things, and don't retort upon him in any case, and don't desert him. Demand from me whatever you like, and don't be tempted to imitate your neighbor (i.e. 'Aisha) in her behavior towards the Prophet), for she (i.e. Aisha) is more beautiful than you, and more beloved to Allah's Apostle.
In those days it was rumored that Ghassan, (a tribe living in Sham) was getting prepared their horses to invade us. My companion went (to the Prophet on the day of his turn, went and returned to us at night and knocked at my door violently, asking whether I was sleeping. I was scared (by the hard knocking) and came out to him. He said that a great thing had happened. I asked him: What is it? Have Ghassan come? He replied that it was worse and more serious than that, and added that Allah's Apostle had divorced all his wives. I said, Hafsa is a ruined loser! I expected that would happen some day.' So I dressed myself and offered the Fajr prayer with the Prophet. Then the Prophet entered an upper room and stayed there alone. I went to Hafsa and found her weeping. I asked her, 'Why are you weeping? Didn't I warn you? Have Allah's Apostle divorced you all?' She replied, 'I don't know. He is there in the upper room.' I then went out and came to the pulpit and found a group of people around it and some of them were weeping. Then I sat with them for some time, but could not endure the situation. So I went to the upper room where the Prophet was and requested to a black slave of his: "Will you get the permission of (Allah's Apostle) for Umar (to enter)? The slave went in, talked to the Prophet about it and came out saying, 'I mentioned you to him but he did not reply.' So, I went and sat with the people who were sitting by the pulpit, but I could not bear the situation, so I went to the slave again and said: "Will you get he permission for Umar? He went in and brought the same reply as before. When I was leaving, behold, the slave called me saying, "Allah's Apostle has granted you permission." So, I entered upon the Prophet and saw him lying on a mat without wedding on it, and the mat had left its mark on the body of the Prophet, and he was leaning on a leather pillow stuffed with palm fires. I greeted him and while still standing, I said: "Have you divorced your wives?' He raised his eyes to me and replied in the negative. And then while still standing, I said chatting: "Will you heed what I say, 'O Allah's Apostle! We, the people of Quraish used to have the upper hand over our women (wives), and when we came to the people whose women had the upper hand over them..."
'Umar told the whole story (about his wife). "On that the Prophet smiled." 'Umar further said, "I then said, 'I went to Hafsa and said to her: Do not be tempted to imitate your companion ('Aisha) for she is more beautiful than you and more beloved to the Prophet.' The Prophet smiled again. When I saw him smiling, I sat down and cast a glance at the room, and by Allah, I couldn't see anything of importance but three hides. I said (to Allah's Apostle) "Invoke Allah to make your followers prosperous for the Persians and the Byzantines have been made prosperous and given worldly luxuries, though they do not worship Allah?' The Prophet was leaning then (and on hearing my speech he sat straight) and said, 'O Ibn Al-Khatttab! Do you have any doubt (that the Hereafter is better than this world)? These people have been given rewards of their good deeds in this world only.' I asked the Prophet . 'Please ask Allah's forgiveness for me. The Prophet did not go to his wives because of the secret which Hafsa had disclosed to 'Aisha, and he said that he would not go to his wives for one month as he was angry with them when Allah admonished him (for his oath that he would not approach Mariyah). When twenty-nine days had passed, the Prophet went to Aisha first of all. She said to him, 'You took an oath that you would not come to us for one month, and today only twenty-nine days have passed, as I have been counting them day by day.' The Prophet said, 'The month is also of twenty-nine days.' That month consisted of twenty-nine days. 'Aisha said, 'When the Divine revelation of Choice was revealed, the Prophet started with me, saying to me, 'I am telling you something, but you needn't hurry to give the reply till you can consult your parents." 'Aisha knew that her parents would not advise her to part with the Prophet . The Prophet said that Allah had said:--
'O Prophet! Say To your wives; If you desire The life of this world And its glitter, ... then come! I will make a provision for you and set you free In a handsome manner. But if you seek Allah And His Apostle, and The Home of the Hereafter, then Verily, Allah has prepared For the good-doers amongst you A great reward.' (33.28) 'Aisha said, 'Am I to consult my parents about this? I indeed prefer Allah, His Apostle, and the Home of the Hereafter.' After that the Prophet gave the choice to his other wives and they also gave the same reply as 'Aisha did."
NOTICE HOW THIS HADITH DOESN'T MENTION ANYTHING ON HAFSA WALKING IN ON PROPHET MUHAMMAD AND HAFSA. IT JUST STATES THAT THE PROPHET WOULD STAY AWAY FROM MARY THE COPT. AND REMEMBER BUKHARI IS THE MOST AUTHENTIC HADITH COLLECTION:
This leads us to our third point. It is often claimed that al-Bukhari was the most careful collector of Muslim traditions, omitting thousands of hadiths that did not meet his strict specifications of authenticity. To help illustrate this fact we quote from the Muslim translator of Al-Bukhari's hadith collection:
It has been UNANIMOUSLY AGREED that Imam Bukhari's work is the most authentic of all the other works in Hadith literature PUT TOGETHER. The authenticity of Al-Bukhari's work is such that the religious learned scholars of Islam said concerning him: "The most authentic book after the Book of Allah (i.e., Al-Qur'an) is Sahih Al-Bukhari."
.Before he recorded each Hadith he would make ablution and offer two Rak'at prayer and supplicate his Lord (Allah). Many religious scholars of Islam tried to find fault in the great remarkable collection- Sahih Al-Bukhari, BUT WITHOUT SUCCESS. It is for this reason, they UNANIMOUSLY AGREED that the most authentic book after the Book of AllahIS Sahih Al-Bukhari. (Translation of the Meanings of Summarized Sahih Al-Bukhari, Arabic-English, translated by Dr. Muhammad Taqi-ud-Din Al-Hilali, Islamic University, Al-Madina Al-Munawwara, Kingdom of Saudi Arabia; compilation: Al-Imam Zain-ud-Din Ahmad bin Abdul-Lateef Az-Zubaidi [Maktaba Dar-us-Salam Publishers & Distributors, Riyadh-Saudi Arabia, 1994], pp. 18-19; bold and capital emphasis ours)
Now, the Prophet's (peace be upon him) wives at then were astonished with the worldly pressures. What they wanted was the spoils of war, so that they could lead a luxurious life like the Kings, hence Umar Ibn Al-Khattab says this:
"I said (to Allah's Apostle) "Invoke Allah to make your followers prosperous for the Persians and the Byzantines have been made prosperous and given worldly luxuries, though they do not worship Allah?' The Prophet was leaning then (and on hearing my speech he sat straight) and said, 'O Ibn Al-Khatttab! Do you have any doubt (that the Hereafter is better than this world)? These people have been given rewards of their good deeds in this world only.' I asked the Prophet".
So Allah SWT revealed the Ayat:
(O Prophet! Say unto thy wives: "If ye desire the world's life and its adornment, come! I will content you and will release you with a fair release. But if ye desire Allah and His Messenger and the abode of the Hereafter, then lo! Allah hath prepared for the good among you an immense reward") (Al-Ahzab 33:28-29).
The Hadith Sina posted is true, and The Prophet (peace be upon him) wives did wish for the worldly gains, The Prophet (peace be upon him) gave them the choice as stated in the Ayat above, but they always chose the Prophet Muhammad (peace be upon him). Yusuf Ali even confirms this by saying :
"We now come to the subject of the position of the Consorts of Purity, the wives of the holy Prophet. Their position was not like that of ordinary women or ordinary wives. They had special duties and responsibilities. The only youthful marriage of the Holy Prophet was his first marriage- that with Hadhrat Khadija, the best of women and the best of wives. He married her fifteen years before he received his call to Apostleship; their married life was twenty-five years and their mutual devotion was of the noblest, judged by spiritual as well as social standards. During her life he had no other wife which was unusual for a man of his standing among his people. When she died his age was 50 and but for two considerations, he would probably never have married again, as he was most abstemious in his physical life. The two considerations which governed his marriage later were : (1) compassion and clemency, as when he wanted to provide for suffering widows, who could not be provided for in any other way in that stage of society; some of them like Sauda had issue by their former marriage, requiring protection; (2) help in his duties of leadership with women who had to be instructed and kept together in the large Muslim family, where women and men had similar social rights. Hadhrat 'Aisha, daughter of Hadhrat Abu Bakr, was clever and learned, and in Hadith she is an important authority on the life of the Prophet. Hadhrat Zainab, daughter of Khuzaima was specially devoted to the poor : she was called the "Mother of the Poor: . The other Zainab, daugher of Jahsh also worked for the poor, for whom she provided from the proceeds of her manual work, as she was skilful in leather in leather work. But all the consorts in their high positions had to work and assist as Mother of the Ummat. Theirs were not idle lives, like those of Odalisques, either for their own pleasure or the pleasure of their husband. They are told here that they had no place in the sacred household if they merely wished for ease or worldly glitter. If such was the case, they would be divorced and amply provided for."
Further more, Yusuf Ali states:
"They were all well-doers. But being in their exalted position they had extra responsibility, and they had to be specially careful to discharge it. In the same way their reward would be "great", for higher services bring higher spiritual satisfaction, though they were asked to deny themselves of the ordinary indulgences of this life".
(Source: The Holy Qur'an , text, translation and commentary by Abdullah Yusuf Ali, footnotes # 3706, and 3707)
Maududi states the same...
the Prophet abstained from visting his wives for nearly a month after the whole incident with Aisha and Hafsa."The discourse contained in vv. 28-35 consists of two parts. In the first part, Allah has given a notice to the wives of the Holy Prophet, who were being impatient of the straitened circumstances, to the effect:" Choose between the world and its adornments, and Allah, His Prophet and the Hereafter. If you seek the former, you should say so openly: you will not be kept back in hardship even for a day, but will be sent off gracefully. And if you seek the latter, you should cooperate with Allah and His messenger and bear patiently." In the second part, initial steps were taken towards the social reforms whose need was being felt by the minds moulded in the Islamic pattern themselves. In this regard, reform was started from the house of the Holy Prophet himself and his wives were commanded to avoid behaving and conducting themselves in the ways of the pre Islamic days of ignorance, ,to remain in their houses with dignity, and to exercise great caution in their conversation with the other men. This was the beginning of the Commandments of Purdah." (Syed Maududi, Commentary on Surah Al Ahzab, Source)