Sahih Bukhari Volume 007, Book 062, Hadith Number 036.
Narrated By 'Aisha : (The wife of the Prophet) that while Allah's Apostle was with her, she heard a voice of a man asking permission to enter the house of Hafsa. 'Aisha added: I said, "O Allah's Apostle! This man is asking permission to enter your house." The Prophet said, "I think he is so-and-so," naming the foster-uncle of Hafsa. 'Aisha said, "If so-and-so," naming her foster uncle, "were living, could he enter upon me?" The Prophet said, "Yes, for foster suckling relations make all those things unlawful which are unlawful through corresponding birth (blood) relations."