About “Your mothers who nursed you...” (Nisa: 23) and the Prophet’s saying: “What is unlawful by lineage is unlawful by nursing.”

Narrated Ibn Abbas (may Allah be pleased with them) that someone said to the Prophet (peace and blessings of Allah be upon him): Why do you not marry (the leader of the martyrs, Sayyiduna) Hamza's (may Allah be pleased with him) daughter? The Prophet (peace and blessings of Allah be upon him) replied: "She is my niece through breastfeeding."

Narrated from Ummul Momineen Aisha Siddiqa (may Allah be pleased with her) that she heard the voice of a man who was seeking permission to enter the house of Ummul Momineen Hafsa (may Allah be pleased with her). Ummul Momineen Aisha (may Allah be pleased with her) said: "O Messenger of Allah, indeed this (non-mahram) man wants to enter your house." So the Prophet Muhammad (peace and blessings be upon him) said: "I know that this is so-and-so, who is Hafsa's (may Allah be pleased with her) foster uncle." Aisha (may Allah be pleased with her) asked: "If so-and-so, who was my uncle through milk relationship, were alive, would I appear before him?" The Prophet (peace and blessings be upon him) said: "Yes! The relationships that are forbidden by lineage are also forbidden by suckling (milk relationship)."

Umm al-Mu’minin Umm Habibah bint Abi Sufyan (may Allah be pleased with her) says: I said, “O Messenger of Allah! Marry my sister, the daughter of Abu Sufyan.” He asked, “Do you like that?” (Do you not dislike having a co-wife?) I said, “Yes.” (But) even now, I am not your only wife, and I do not dislike my sister sharing goodness with me.” He said, “It is not lawful for me (to have two sisters in marriage at the same time).” I said, “We have heard that you want to marry the daughter of Abu Salamah?” The Prophet (peace be upon him) asked, “The daughter of Umm Salamah? (Do you mean I want to marry her?)” I said, “Yes.” So the Prophet (peace be upon him) said, “Even if she were not my stepdaughter (the daughter of my wife from a previous husband), she would still not be lawful for me, because she is my niece through breastfeeding. Both Abu Salamah (her father) and I were breastfed by Thuwaibah (may Allah be pleased with her). (O lady!) Do not present your sisters and daughters to me (they are not lawful for me).”