Jami` at-Tirmidhi 1116

Abu Burdah bin Abi Musa narrated from his father that the Messenger of Allah said:"Three will receive their reward twice: A slave who fulfills the rights of Allah and the rights of his owners, then he will be given his reward twice. And a man who has a beautiful slave girl, so he teaches her good manners, then he frees her, then he married her seeking the Face of Allah by that; then he will be given his reward twice. And a man who believed in an earlier Book, then another Book came to him and he believed in it; then he will be given his reward twice