Full Sharh — Riyad as-Salihin 686
وعنه قال: قال رسول الله صلى الله عليه وسلم : "ثلاثة لهم أجران: رجل من أهل الكتاب آمن بنبيه، وآمن بمحمد، والعبد المملوك إذا أدى حق الله، وحق مواليه، ورجل كانت له أمة فأدبها فأحسن تأديبها وعلمها فأحسن تعليمها، ثم أعتقها فتزوجها فله أجران" ((متفق عليه))
English Translation
Abu Musa Al-Ash'ari (May Allah be pleased with him) reported:: The Messenger of Allah (ﷺ) said, "Three kinds of people will have a double reward: A man from the People of the Book who believes in his Prophet and (also) believes in Muhammad; a slave who discharges properly the duties towards Allah and towards his master; and a man who possesses a slave-girl and teaches her manners, educates her well, and frees her and then marries her.".
Source
Riyad as-Salihin(رياض الصالحين)
The Book of Jihad
Hadith Number
686