Ash'ath bin Abi al-Sha'tha' narrates from his father that he said: I heard Masruq saying: I heard Sayyidah Aishah (may Allah be pleased with her) saying: The Messenger of Allah (peace and blessings be upon him) entered upon me while a man was with me, and I saw displeasure on 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. She said: He (peace and blessings be upon him) said: "Look at who has nursed you, for fosterage is established only through (nursing that) removes hunger." And he said: Bishr said in his narration: "Look at whether they are (truly) your brothers." Bukhari and Muslim recorded it in their Sahihs from the hadith of Shu'bah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرضاع / 15652