It is narrated from Sayyiduna Ali (may Allah be pleased with him), he says: I said, "O Messenger of Allah! You prefer kinship among the Quraysh and leave us, that is, Banu Hashim." The Prophet (peace and blessings be upon him) said: "Do you have any kinship?" I said: "Yes, there is the daughter of Sayyiduna Hamza (Salma)." The Prophet (peace and blessings be upon him) said: "She is not lawful for me, because she is my foster niece."
Hadith Takhrij«أخرجه مسلم: 1446، (انظر مسند أحمد ترقيم الرسالة: 620 ترقیم بيت الأفكار الدولية: 620»
Brief Explanation
Benefits: … Abu Lahab’s slave woman nursed the Noble Prophet (sallallahu alayhi wa sallam) and Sayyiduna Hamzah (radi Allahu anhu), thus the Prophet (sallallahu alayhi wa sallam) and Sayyiduna Hamzah (radi Allahu anhu) became foster brothers, and each one’s children became unmarriageable kin (mahram) to the other. If only lineage were considered, the Prophet (sallallahu alayhi wa sallam) could have married Sayyiduna Hamzah’s daughter, but fosterage (rada‘ah) became an impediment.