Sahih Bukhari | Volume 1 | Hadith Number 97A

Narrated by Abu Burda's father
Allah's Apostle 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 (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."