Sahih Bukhari | Volume 7 | Hadith Number 39
Narrated by 'Aisha
That the Prophet entered upon her while a man was sitting with her. Signs of anger 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."