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].