Masruq narrates from Aishah (may Allah be pleased with her) that she said: The Messenger of Allah (peace and blessings be upon him) entered upon me while a man was with me. He (peace and blessings be upon him) said: "O Aishah, who is this?" I said: He is my foster brother. He (peace and blessings be upon him) said: "O Aishah, look carefully at who your foster brothers are, for fosterage is established only through (nursing that) removes hunger." Imam Bukhari (may Allah have mercy on him) recorded it in his Sahih from Muhammad bin Kathir (may Allah have mercy on him), and Imam Muslim (may Allah have mercy on him) recorded it in the Sahih through two other chains from Sufyan (may Allah have mercy on him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرضاع / 15651