Musnad Ahmad 25:1285

Muhammad ibn Ja'far and Ibn Bakr 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 to marry the daughter of Hamzah. He said: "She is the daughter of my brother through breastfeeding, for indeed, breastfeeding prohibits what lineage prohibits."