Sayyidah Maymunah (may Allah be pleased with her), who was (married) to the Messenger of Allah (peace and blessings be upon him), had a black slave girl. She said: O Messenger of Allah, I intend to free her. The Messenger of Allah (peace and blessings be upon him) said: "Why do you not use her instead to free your brother's daughter or your sister's daughter from herding sheep?"
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20364