Sahih Adab al-Mufrad

It is narrated from Sayyiduna Abu Musa (may Allah be pleased with him) that he said: The Messenger of Allah (peace and blessings of Allah be upon him) said to him: "There are three types of people who will have a double reward: A man from the People of the Book who believed in his Prophet and also believed in Muhammad (peace be upon him), for him there are two rewards; and a slave who fulfilled the right of Allah and the right of his masters (and in one narration: fulfilled the right of his king who owns him); and a man who had a slave-girl with whom he fulfilled the rights of marriage, taught her discipline and taught her good discipline, and educated her and gave her good education, then set her free and married her, for him there are two rewards." ‘Amir said: We have given you this hadith for free, that is, without anything in exchange, whereas for a hadith of lesser rank, one would have to travel as far as Madinah.