Bukhari Collection: ::Book Number 4:: Hadith # 337
Narrated `Amra bint `Abdur-Rahman:`Aisha, the wife of the Prophet told her that once Allah's Apostle was with her and she heard somebody asking permission to enter Hafsa's house. She said, "O Allah's Apostle! This man is asking permission to enter your house." Allah's Apostle replied, "I think he is so-and-so (meaning the foster uncle of Hafsa). What is rendered illegal because of blood relations, is also rendered illegal because of the corresponding foster-relations."