Abu Musa Al-Ash'ari (May Allah be pleased with him) reported:
The Messenger of Allah (ﷺ) said, "Three kinds of people will have a double reward: A man from the People of the Book who believes in his Prophet and (also) believes in Muhammad; a slave who discharges properly the duties towards Allah and towards his master; and a man who possesses a slave-girl and teaches her manners, educates her well, and frees her and then marries her."[Al-Bukhari and Muslim].
App reference: Book 12, Hadith 81
Arabic/English book reference: Book 12, Hadith 1365