that the Prophet (ﷺ) 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 established only when milk is the only food of the child."
Sahih al-Bukhari Book 67, Hadith 40
← Previous Hadith (daughter of Abu Sufyan) I said, "O Allah's Messenger (ﷺ)! Marry my sister. the daughter of Abu Sufyan." The Prophet (ﷺ) 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 (ﷺ) 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 'Urwa: 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."

Sahih al-Bukhari Book 67, Hadith 39 Next Hadith → that Aflah the brother of Abu Al-Qu'ais, her foster uncle, came, asking permission to enter upon her after the Verse of Al-Hijab (the use of veils by women) was revealed. `Aisha added: I did not allow him to enter, but when Allah's Messenger (ﷺ) came, I told him what I had done, and he ordered me to give him permission.

Sahih al-Bukhari Book 67, Hadith 41