Sahih Bukhari Volume 7, Book 62, Hadith Number 166.

Narrated By ‘Aisha : My foster uncle came and asked permission (to enter) but I refused to admit him till I asked Allah’s Apostle about that. He said, “He is your uncle, so allow him to come in.” I said, “O Allah’s Apostle! I have been suckled by a woman and not by a man.” Allah’s Apostle said, “He is your uncle, so let him enter upon you.” And that happened after the order of Al-Hijab (compulsory veiling) was revealed. All things which become unlawful because of blood relations are unlawful because of the corresponding foster suckling relations.