It is narrated from Sayyiduna Abu Umamah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Four (kinds of people) will be given their reward twice: the wives of the Messenger of Allah (peace and blessings be upon him), whoever of the People of the Book accepts Islam, a man who had a slave woman who pleased him, so he freed her and then married her, and a slave who fulfils the right of Allah and the right of his masters."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7825