Musnad Ahmad 5:788

Muhammad ibn Ubayd narrated to us, al-A’mash narrated to us, from Sa’d ibn Ubaydah, from Abu Abd al-Rahman, from Ali, may Allah be pleased with him, who said: I said: "O Messenger of Allah, why do you choose from Quraysh and do not marry among us?" He said: "And do you have anything?" I said: "Yes, the daughter of Hamzah." He said: "That is the daughter of my brother through breastfeeding."