Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrated that a man came to the Prophet (peace and blessings be upon him) and said: "I am obliged to free a slave, and I have a black, non-Arab slave girl." He (peace and blessings be upon him) said: "Bring her to me." He (peace and blessings be upon him) asked: "Do you bear witness that there is no true god except Allah?" She said: "Yes." He (peace and blessings be upon him) asked: "Do you bear witness that I am the Messenger of Allah?" She said: "Yes." He (peace and blessings be upon him) said: "Then free her."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12317