The Prophet (peace and blessings be upon him) said: "Three kinds of people are given their reward twice: a man from the People of the Book who believed in his prophet, and when he found the Prophet (peace and blessings be upon him), he believed in him too, followed him and affirmed him; for him there are two rewards. And a slave who fulfils the right of Allah Almighty and also fulfils the right of his master; for him too there are two rewards. And a man who has a slave woman, whom he brings up and brings up well, then teaches her manners and trains her well, then frees her and marries her; for him too there are two rewards."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13738