Sahih Bukhari Volume 3, Book 48, 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).”