Hadith: Three types of people will receive a double reward: a man from the People of the Book who believed in his Prophet and then believed in Muhammad, a slave who fulfilled his duties towards Allah and towards his masters, and a man who had a female slave and he taught her good manners, educated her well, then freed her and married her; he will receive a double reward

عن أبي موسى الأشعري رضي الله عنه مرفوعاً: «ثلاثة لهم أجْرَان: رجُلٌ من أهل الكتاب آمن بِنَبِيِّه، وآمَن بمحمد، والعَبْد المملوك إذا أَدَّى حَقَّ الله، وحَقَّ مَوَالِيه، ورجل كانت له أمَة فأدَّبَها فأحسن تَأدِيبَها، وَعَلَّمَهَا فأحسن تَعْلِيمَهَا، ثم أعْتَقَها فتزوجها؛ فله أجران».
[صحيح] - [رواه البخاري]

Abu Mūsa al-Ash‘ari (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "Three types of people will receive a double reward: a man from the People of the Book who believed in his Prophet and then believed in Muhammad, a slave who fulfilled his duties towards Allah and towards his masters, and a man who had a female slave and he taught her good manners, educated her well, then freed her and married her; he will receive a double reward."
[Authentic hadith] - [Narrated by Bukhari]

Explanation

Three types of people will have their reward doubled on the Day of Judgment: 1. A man from the People of the Book; i.e. a Jew or a Christian, who believed in his Prophet, Mūsa or ‘Īsa (peace be upon them), before Prophet Muhammad (may Allah's peace and blessings be upon him) was sent and before his call reached him. Then when Prophet Muhammad was sent and his call to Islam reached him, he believed in him. Such a person will receive two rewards; one for his belief in his messenger who was sent to him first, and the second for his belief in Muhammad (may Allah's peace and blessings be upon him). 2. A slave who worships Allah, the Almighty, and carries out the duties that his master assigns to him in the best manner will receive two rewards. 3. A man who had a female slave, and he raised her well and taught her the matters of her religion, the lawful and the unlawful thereof, then he freed her and married her. He will receive two rewards; one for teaching her and then freeing her, and the second for his kindness to her after freeing her by marrying her and thus sufficing her from need and helping her preserve her chastity.