"The Messenger of Allah said: 'There are three who will be given a twofold reward: A man who has a slave woman whom he disciplines and disciplines her well, and teaches and teaches her well, then he manumits her and marries her; a slave who fulfills his duty toward Allah and toward his masters; and a believer from among the People of the Book.'"
Sunan an-Nasa'i Book 26, Hadith 149