Musnad Ahmad 5:532

Waki’ narrated to us, Sufyan narrated to us, from ‘Ali ibn Zayd, from Sa’id ibn al-Musayyib, who said: ‘Ali said: I said to the Messenger of Allah, peace and blessings of Allah be upon him, "Shall I not guide you to the most beautiful young woman in Quraysh?" He said, "And who is she?" I said, "The daughter of Hamzah." He said, "Do you not know that she is the daughter of my brother through breastfeeding? Indeed, Allah has made unlawful through breastfeeding what He has made unlawful through lineage."