Musannaf Ibn Abi Shayba 35:126

Hushaym narrated to us, from Abd al-Aziz ibn Suhayb, from Anas ibn Malik, that the Prophet (may Allah bless him and grant him peace) emancipated Safiyyah and married her. He said: It was said to him, "What did he give her as a dowry?" He said: "He gave her herself as a dowry; he made her emancipation her dowry."