Narrated Abu Burda's father:

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 (ﷺ) Muhammad (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."