So Cool Sunny's answer is good.
And to the stupid Shiites, here is a verse from the Quran
Allah says in the Quran:
“O Prophet! Say to your wives and your daughters and the women of the believers that they let down upon them their over-garments; this will be more proper, that they may be known, and thus they will not be given trouble; and Allah is Forgiving, Merciful.” (Quran, 33:59)
Here, Allah uses the plural word for daughters, not the singular form. Allah uses the term “banaatuka” which means “your daughters.” If it was one daughter only, it would be “bintuka.”
Also read this in al-islam.org a famous Shiite site. It says in the answer
"Most reports (like those of Ibn Ishaq, al-Mas'udi) indicate that she was
the mother of all the Prophet's children apart from Ibrahim (who was the
son of Mary the Copt). She bore the following children to the Prophet:
al-Qasim; Zaynab, Ruqqayya Umm Kulthum and Hazrat Fatima (A.S)."
http://www.al-islam.org/organizations/aalimnetwork/msg00083.html
Also we read in Nahjul Balagha the following
Nahjul Balagha, Sermon 163
(Ali (ra) went to see Uthman (ra) and said:
“You have seen as we have seen and you have heard as we have heard. You sat in the company of the Prophet of Allah as we did. (Abu Bakr) Ibn Abi Quhafah and (`Umar) ibn al-Khattab were no more responsible for acting righteously than you, since you are nearer than both of them to the Prophet of Allah through kinship, and you also hold relationship to him by marriage which they do not hold.”