Musannaf Ibn Abi Shayba 7:491

Abdah narrated to us, from Salih ibn Hayy, from al-Sha'bi, from Abu Burdah, from Abu Musa, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Whoever has a slave girl, disciplines her and perfects her discipline, and teaches her and perfects her education, then frees her and marries her, he shall have two rewards."