Musnad Ahmad 25:1184

'Affan narrated to us, Hammam narrated to us, Qatadah narrated to us, from Jabir ibn Zayd, from Ibn 'Abbas: That the Prophet, peace and blessings of Allah be upon him, was urged to marry the daughter of Hamzah. He said: "She is the daughter of my brother through breastfeeding, and she is not lawful for me. And that which is forbidden by breastfeeding is forbidden by kinship."