Abu Musa Ashari reported,
The Prophet ﷺ said, "Three will receive a double reward. A man who has a slave-girl whom he teaches, and teaches her well, disciplines, and disciplines her well, then frees her and marries her, he will have two rewards. A slave who fulfills the right of Allah 
and the right of his masters, he too will have two rewards. And a man from the People of the Book who believed in what Isa brought and then believed in what Muhammad ﷺ brought, he will have two rewards."
Ahmad 19532
Sahihحدثنا عبد الرحمن عن سفيان عن صالح الثوري عن الشعبي عن أبي بردة
عن أبي موسى
عن النبي ﷺ قال من كانت له أمة فعلمها فأحسن تعليمها وأدبها فأحسن تأديبها وأعتقها فتزوجها فله أجران وعبد أدى حق الله عز وجل وحق مواليه ورجل من أهل الكتاب آمن بما جاء به عيسى وما جاء به محمد ﷺ فله أجران
Its isnad is sahih. Salih Thawri is Salih bin Salih bin Hayy, who is trustworthy, and his hadith is accepted by the group of scholars. The hadith was narrated by Bukhari, 4/174, in the Book of Wills, chapter on the merit of one who embraces Islam from the People of the Book, and Muslim, 1/134, no. 241, and Tirmidhi, 2/415, no. 1116, who said it is hasan sahih, and Nasai, 6/115, in the Book of Marriage, chapter on a man freeing his slave-girl and then marrying her.