Mukhtasar Bukhari

Umm al-Mu'mineen Aisha Siddiqa (may Allah be pleased with her) narrates that the Prophet Muhammad (peace and blessings be upon him) came to her, and at that time a man was sitting with her. Seeing this, the blessed face of the Prophet (peace be upon him) changed, as if he disliked it. I said, "This is my foster brother." So the Prophet (peace be upon him) said: "Consider carefully who your brothers are, because the relationship of fosterage is only established when the child’s nourishment (is milk)."