Masruq narrates from Sayyidah Aishah (may Allah be pleased with her): The Messenger of Allah (peace and blessings be upon him) came in to me while a man was sitting with me. This weighed heavily on him (peace and blessings be upon him), and I saw anger in his (peace and blessings be upon him) face. I said: "O Messenger of Allah (peace and blessings be upon him), he is my foster brother." He (peace and blessings be upon him) said: "Look carefully at who your foster brothers are, for suckling is (only that which satisfies) hunger." (That is, drinking milk to fullness five times within the age of two years.) Muslim narrated it in his Sahih from Hannad bin al-Sari from Abu al-Ahwas, and both of them recorded it from the hadith of al-Thawri and Shu'bah from Ash'ath.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرضاع / 15633