From Sahih Bukhari (Hadith 5083):
Narrated Abu Burda's father:Allah's Messenger (ﷺ) said, any man who has a slave girl whom he educates properly, teaches good manners, manumits and marries her, will get a double reward And if any man of the people of the Scriptures believes in his own prophet and then believes in me too, he will (also) get a double reward And any slave who fulfills his duty to his master and to his Lord, will (also) get a double reward
Reference: Bukhari 5083