Musnad Ahmad 32:1601

Surayj ibn al-Nu‘man narrated to us, Hammad—meaning ibn Zayd—narrated to us, from Thabit and ‘Abd al-‘Aziz ibn Suhayb, from Anas ibn Malik, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, emancipated Safiyyah, and he made her emancipation her dowry."