Allah's Messenger (ﷺ) said, "If a slave is honest and faithful to his master and worships his Lord (Allah) in a perfect manner, he will get a double reward."
Sahih al-Bukhari Book 49, Hadith 29
← Previous Hadith I saw Abu Dhar Al-Ghifari wearing a cloak, and his slave, too, was wearing a cloak. We asked him about that (i.e. how both were wearing similar cloaks). He replied, "Once I abused a man and he complained of me to the Prophet (ﷺ) . The Prophet (ﷺ) asked me, 'Did you abuse him by slighting his mother?' He added, 'Your slaves are your brethren upon whom Allah has given you authority. So, if one has one's brethren under one's control, one should feed them with the like of what one eats and clothe them with the like of what one wears. You should not overburden them with what they cannot bear, and if you do so, help them (in their hard job).

Sahih al-Bukhari Book 49, Hadith 28 Next Hadith → The Prophet (ﷺ) said, "He who has a slave-girl and teaches her good manners and improves her education and then manumits and marries her, will get a double reward; and any slave who observes Allah's right and his master's right will get a double reward."

Sahih al-Bukhari Book 49, Hadith 30