Musnad Abu Ya'la al-Mawsili 3:121

Ishaq ibn Isma'il narrated to us, Waki' narrated to us, from Sufyan, from Ali ibn Zayd, from Sa'id ibn al-Musayyib, from Ali, who said: I said, "O Messenger of Allah, shall I not guide you to the most beautiful young woman in Quraysh?" He said, "And who is she?" I said, "The daughter of Hamza." He said, "Do you not know that she is the daughter of my brother through breastfeeding? And indeed, Allah has made forbidden through breastfeeding what He has made forbidden through lineage."