Hadith #97 (Sahih al-Bukhari)
Book: Knowledge
Narrator(s): Abu Burda's father
Grade: Sahih

Allah’s Messenger (ﷺ) said “Three persons will have a double reward:
 1. A Person from the people of the scriptures who believed in his prophet (Jesus or Moses) and then believed in the Prophet (ﷺ) Muhammad (i .e. has embraced Islam).
 2. A slave who discharges his duties to Allah and his master.
 3. A master of a woman-slave who teaches her good manners and educates her in the best possible way (the religion) and manumits her and then marries her.”

Source: https://parallelquran.com/hadith/sahih-al-bukhari-97/

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Hadith #2889 (Sahih al-Bukhari)
Book: Fighting for the Cause of Allah (Jihaad)
Narrator(s): Abu Burda's father
Grade: Sahih

The Prophet (ﷺ) said, “Three persons will get their reward twice. (One is) a person who has a slave girl and he educates her properly and teaches her good manners properly (without violence) and then manumits and marries her. Such a person will get a double reward. (Another is) a believer from the people of the scriptures who has been a true believer and then he believes in the Prophet (ﷺ) (Muhammad). Such a person will get a double reward. (The third is) a slave who observes Allah’s Rights and Obligations and is sincere to his master.”

Source: https://parallelquran.com/hadith/sahih-al-bukhari-2889/

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Hadith #4879 (Sahih al-Bukhari)
Book: Wedlock, Marriage (Nikaah)
Narrator(s): Abu Burda's father
Grade: Sahih

Allah’s Messenger (ﷺ) said, any man who has a slave girl whom he educates properly, teaches good manners, manumits and marries her, will get a double reward And if any man of the people of the Scriptures believes in his own prophet and then believes in me too, he will (also) get a double reward And any slave who fulfills his duty to his master and to his Lord, will (also) get a double reward.”

Source: https://parallelquran.com/hadith/sahih-al-bukhari-4879/