Sayyiduna Abu Musa (may Allah be pleased with him) narrated that the Messenger of Allah (peace and blessings be upon him) said: "Whoever has a slave-girl and teaches her good manners and gives her good education, then frees her and marries her, has a double reward; and the slave who fulfils the right of Allah and the right of his master will also have a double reward."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النفقات / 15802