Musnad Ahmad 785:2

'Abd al-Samad narrated to us, Hammad ibn Salamah narrated to us, Muhammad ibn 'Amr narrated to us, from Abu Salamah, from al-Sharid, that his mother bequeathed that a believing slave be freed on her behalf. He asked the Messenger of Allah (peace and blessings of Allah be upon him) about that, and said: "I have a black Nubian slave girl; shall I free her on her behalf?" He said: "Bring her." So I called her, and she came. He said to her: "Who is your Lord?" She said: "Allah." He said: "Who am I?" She said: "The Messenger of Allah." He said: "Free her, for she is a believer."