Ishaq bin Sulaiman al-Razi إسحاق بن سليمان الرازي · The Chapters on Marriage

that the Messenger of was offered the daughter of Hamzah bin 'Abdul-Muttalib in marriage, and he said: “She is the daughter of my brother through breastfeeding, and breastfeeding makes unlawful (for marriage) the same things that blood ties make unlawful.”