From Sahih Bukhari (Hadith 3446):
Narrated Abu Musa Al-Ash`ari:Allah's Messenger (ﷺ) said, "If a person teaches his slave girl good manners properly, educates her properly, and then manumits and marries her, he will get a double reward. And if a man believes in Jesus and then believes in me, he will get a double reward. And if a slave fears his Lord (i.e. Allah) and obeys his masters, he too will get a double reward
Reference: Bukhari 3446