Bukayr bin 'Abdullah bin al-Ashj بكير بن عبد الله بن الأشج · The Book of Marriage

"We have been saying that you want to marry Durrah bint Abi Salamah." The Messenger of Allah said: "As a co-wife to Umm Salamah? Even if I were not married to Umm Salamah, she would not be permissible to me, for her father is my brother through breast-feeding."

"I said: 'O Messenger of Allah, there is a man asking permission to enter your house.' The Messenger of Allah said: 'I think it is so-and-so the paternal uncle of Hafsah through breast-feeding.' 'Aishah said: If so-and-so (her own paternal uncle through breast-feeding) were alive, would he be allowed to enter upon me?' The Messenger of Allah said: 'What becomes unlawful (for marriage) through breast-feeding is that which becomes unlawful through birth.'"