From Sahih Bukhari (Hadith 3011):
Narrated Abu Burda's father:The Prophet (ﷺ) said, "Three persons will get their reward twice. (One is) a person who has a slave girl and he educates her properly and teaches her good manners properly (without violence) and then manumits and marries her. Such a person will get a double reward. (Another is) a believer from the people of the scriptures who has been a true believer and then he believes in the Prophet (ﷺ) (Muhammad). Such a person will get a double reward. (The third is) a slave who observes Allah's Rights and Obligations and is sincere to his master
Reference: Bukhari 3011