Musannaf Ibn Abi Shayba 23:35

Ali bin Hashim narrated to us, from Ibn Abi Layla, from Sa’id bin Jubayr, from Ibn Abbas, from the Prophet (may Allah bless him and grant him peace) that a man came to the Prophet (may Allah bless him and grant him peace) and said: "My mother has an obligation of a believing slave [to be freed], and I have a black, non-Arab slave." He said: "Bring her." He said: "Do you testify that there is no god but Allah and that I am the Messenger of Allah?" She said: "Yes." He said: "Then free her."