Musnad Ahmad 1153:1435

I read to Abd al-Rahman: Malik, from Abdullah ibn Abi Bakr, from Amrah bint Abd al-Rahman, that Aisha informed her that the Messenger of Allah (peace and blessings of Allah be upon him) was with her, and that she heard the voice of a man asking for permission to enter in the house of Hafsa. Aisha said: I said, "O Messenger of Allah, this is a man asking for permission to enter your house." The Messenger of Allah (peace and blessings of Allah be upon him) said: "I think it is so-and-so," for the paternal uncle of Hafsa through breastfeeding. Aisha said: "O Messenger of Allah, if so-and-so were alive—for her paternal uncle through breastfeeding—would he enter upon me?" The Messenger of Allah (peace and blessings of Allah be upon him) said: "Yes, indeed breastfeeding makes prohibited what birth makes prohibited."