Sahih al-Bukhari #97

Allah's Messenger (ﷺ) said "Three persons will have a double reward: 1. A Person from the people of the scriptures who believed in his prophet (Jesus or Moses) and then believed in the Prophet Muhammad (ﷺ) (i.e. has embraced Islam). 2. A slave who discharges his duties to Allah and his master. 3. A master of a woman-slave who teaches her good manners and educates her in the best possible way (the religion) and manumits her and then marries her