Sahih Bukhari | Volume 7 | Hadith Number 20

Narrated by Abu Burda's father
Allah's Apostle 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 fulfils his duty to his master and to his Lord, will (also) get a double reward."