It was narrated from 'Aishah:
that the Prophet entered upon her and there was a man with her. He said: "Who is this? She said: "This is my brother." He said: "Look at whom you allow to enter upon you, because the breastfeeding (that makes a person Mahram) is that which satisfies hunger." (IT)
English reference (IT): Vol. 3, Book 9, Hadith 1945 (IT)
Arabic reference (IT): Book 9, Hadith 2021 (IT)