Yahya related to me from Malik from Abdullah ibn Abi Bakr fromAmra bint Abd ar-Rahman that A'isha, umm al-muminin informed her thatthe Messenger of Allah, may Allah bless him and grant him peace, waswith her and she heard the voice of a man asking permission to enterthe room of Hafsa. A'isha said that she had said, "Messenger of Allah!There is a man asking permission to enter your house!" The Messengerof Allah, may Allah bless him and grant him peace, said, "I think itis so-and-so" (referring to a paternal uncle of Hafsa by suckling).A'isha said, "Messenger of Allah! If so-and-so were alive (referringto her paternal uncle by suckling) could he enter where I am?" TheMessenger of Allah, may Allah bless him and grant him peace, said,"Yes. Suckling makes haram as birth makes haram."
USC-MSA web (English) reference: Book 30, Hadith 1
Arabic reference: Book 30, Hadith 1275