Sahih Bukhari Volume 003, Book 048, Hadith Number 815.
Narated By 'Aisha : Once the Prophet came to me while a man was in my house. He said, "O 'Aisha! Who is this (man)?" I replied, "My foster brothers" He said, "O 'Aisha! Be sure about your foster brothers, as fostership is only valid if it takes place in the suckling period (before two years of age)."