Mishkat al-Masabih 11

Abu Musa al-Ash‘ari reported God’s messenger as saying, "Three types will have a double reward: one of the people of the Book who believes in his prophet and believes in Muhammad; a slave when he fulfils what is due to God and what is due to his patrons; and a man who has a slavegirl with whom he has intercourse, who gives her a good training in manners and a good education, then sets her free and marries her—he will have a double reward.” (Bukhari and Muslim.)