From Sahih Bukhari (Hadith 2547):
Narrated Abu Musa Al-Ash`ari:The Prophet (ﷺ) said, "He who has a slave-girl and teaches her good manners and improves her education and then manumits and marries her, will get a double reward; and any slave who observes Allah's right and his master's right will get a double reward
Reference: Bukhari 2547