Musnad Abu Ya'la al-Mawsili 3:123

Ubayd Allah ibn Umar al-Qawariri narrated to us, Hisham ibn Abd al-Malik narrated to us, Shu'bah narrated to us, from Muhammad ibn Ubayd Allah Abu 'Awn, who said: I heard Abu Salih say: Ali went out and said: "Ask me." Ibn al-Kawwa asked him about the daughter of a brother through breastfeeding. Ali said: "The daughter of Hamzah was mentioned to the Messenger of Allah, may Allah bless him and grant him peace, and he said: 'She is the daughter of my brother through breastfeeding.'"