Musnad Ahmad 5:474

'Abd al-Rahman narrated to us, from Sufyan, from al-A'mash, from Sa'd ibn 'Ubaydah, from Abu 'Abd al-Rahman, from 'Ali, who said: I said: "O Messenger of Allah, why do I see you choosing from among Quraysh and leaving us, so that you do not marry among us?" He said: "And do you have anything?" I said: "The daughter of Hamzah." He said: "She is the daughter of my brother through breastfeeding."