Musnad Ahmad 5:368

Hajjaj narrated to us, Israel narrated to us, from Abu Ishaq, from Hani ibn Hani and Hubayrah ibn Yarim, from Ali, that the daughter of Hamza followed them calling out: "O uncle, O uncle!" Ali reached for her and took her hand, and said to Fatimah: "Take your cousin, and take her with you." Then Ali, Zayd, and Ja'far disputed over her. Ali said: "I took her, and she is my cousin." Ja'far said: "She is my cousin, and her maternal aunt is under my marriage." Zayd said: "She is the daughter of my brother." The Messenger of Allah, peace and blessings of Allah be upon him, judged her to her maternal aunt and said: "The maternal aunt is in the position of the mother." Then he said to Ali: "You are from me, and I am from you." He said to Ja'far: "You resemble my physical appearance and my character." He said to Zayd: "You are our brother and our freedman (mawla)." Ali said to him: "O Messenger of Allah, will you not marry the daughter of Hamza?" He said: "She is the daughter of my brother through breastfeeding."