Musnad Ahmad 5:603

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Abu 'Awn, who said: I heard Abu Salih, who said: 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."