Mukhtasar Bukhari

Narrated from Sayyiduna Abu Musa (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said: There are three persons for whom there is a double reward: (1) A person who is from the People of the Book, who believed in his own prophet and (then) also believed in Muhammad (peace and blessings of Allah be upon him); (2) A slave, when he fulfills the rights of Allah and the rights of his masters; (3) A person who has a slave-girl, teaches her good manners and gives her excellent training, and gives her good and excellent education, then sets her free and marries her, so for him there is a double reward.