Three kinds of people will have a double reward

Three kinds of people will have a double reward: A man from the People of the Scripture (Jews or Christians) who believed in his prophet and (also) believed in Muhammad; a slave who fulfills his duties towards Allah and towards his master; and a man who owned a slave-girl, so he disciplined her well and educated her well then freed her and married her. He will receive a double reward. [Al-Bukhari and Muslim]