The Prophet (peace and blessings be upon him) said: "Any man who trains his slave woman and trains her well, and educates her and educates her well, then frees her and marries her, for him there are two rewards; and any slave who fulfils the right of his Lord and also fulfils the right of his master, for him too there are two rewards."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13739