Musnad Ahmad 32:2054

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from ‘Abd al-‘Aziz, from Anas, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, married Safiyyah." Thabit said to him: "What did he give her as a dowry?" He said: "He gave her herself as a dowry; he freed her and married her."