Musnad Ahmad 25:1378

Yahya narrated to us, from Shu'bah, Qatadah narrated to us, from Jabir ibn Zayd, from Ibn Abbas, who said: It was said to the Prophet, peace and blessings of Allah be upon him: "If only you would marry the daughter of Hamza?" He said: "She is the daughter of my brother through breastfeeding."