Sahih Bukhari Volume 7, Book 62, Hadith Number 38.

Narrated By Um Habiba : (Daughter of Abu Sufyan) I said, “O Allah’s Apostle! Marry my sister. the daughter of Abu Sufyan.” The Prophet said, “Do you like that?” I replied, “Yes, for even now I am not your only wife and I like that my sister should share the good with me.” The Prophet said, “But that is not lawful for me.” I said, We have heard that you want to marry the daughter of Abu Salama.” He said, “(You mean) the daughter of Um Salama?” I said, “Yes.” He said, “Even if she were not my step-daughter, she would be unlawful for me to marry as she is my foster niece. I and Abu Salama were suckled by Thuwaiba. So you should not present to me your daughters or your sisters (in marriage).” Narrated ‘Ursa; Thuwaiba was the freed slave girl of Abu Lahb whom he had manumitted, and then she suckled the Prophet. When Abu Lahb died, one of his relatives saw him in a dream in a very bad state and asked him, “What have you encountered?” Abu Lahb said, “I have not found any rest since I left you, except that I have been given water to drink in this (the space between his thumb and other fingers) and that is because of my manumitting Thuwaiba.”