Musnad Ahmad 788:226

'Abdah ibn Sulayman narrated to us, he said: Salih ibn Salih narrated to us, from al-Sha'bi, from Abu Burdah, from Abu Musa al-Ash'ari, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever has a slave girl and disciplines her and perfects her discipline, and teaches her and perfects her teaching, then frees her and marries her, he shall have two rewards. And any man from the People of the Book who believes in his prophet and believes in Muhammad, he shall have two rewards. And any owned slave who fulfills the right of Allah (Mighty and Majestic is He) upon him and the right of his masters, he shall have two rewards."