Hadiths 7.38 to 7.39

7.38:
Narrated 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."

7.39:
Narrated `Aisha:
that the Prophet entered upon her while a man was sitting with
her. Signs of answer seemed to appear on his face as if he
disliked that. She said, "Here is my (foster) brother." He
said, "Be sure as to who is your foster brother, for foster
suckling relationship is eestablished only when milk is the
only food of the child."