Musnad Ahmad 25:632

Abdullah ibn Bakr and Muhammad ibn Ja'far narrated to us, they said: Sa'id narrated to us, from Qatadah, from Jabir ibn Zayd, from Ibn Abbas, that the Messenger of Allah (peace and blessings of Allah be upon him) was requested regarding the daughter of Hamzah that he should marry her. He said: "She is the daughter of my brother through breastfeeding, and indeed, that which is forbidden by lineage is forbidden by breastfeeding."