Musnad Ahmad 25:633

Abdullah ibn Bakr narrated to us, he said: Sa’id narrated to us, from Ali ibn Zayd, from Sa’id ibn al-Musayyib, from Ibn Abbas, that Ali said to the Prophet, peace and blessings of Allah be upon him, regarding the daughter of Hamza, and he mentioned her beauty. The Messenger of Allah, peace and blessings of Allah be upon him, said: "She is the daughter of my brother through breastfeeding." Then the Prophet of Allah, peace and blessings of Allah be upon him, said: "Do you not know that Allah, the Almighty and Majestic, has made forbidden through breastfeeding what He has made forbidden through lineage?"