Musnad Abu Ya'la al-Mawsili 130:39

'Abd al-A'la narrated to us, 'Uthman ibn 'Umar narrated to us, Shu'bah informed us, 'Abd al-'Aziz ibn Suhayb informed us, he said: I heard Anas ibn Malik mention the marriage of the Prophet, may Allah bless him and grant him peace, to Safiyyah. Thabit said: "What did he give her as a dowry?" Anas said: "He gave her herself as a dowry; he emancipated her and married her."