Ibn Umar reported Allah's Messenger (ﷺ) as saying
When a slave looks to the welfare of his master and worships Allah well, he has two rewards for him.
This hadith has been narrated on the authority of Ibn 'Umar through another chain of transmitters.
Abu Huraira reported Allah's Messenger (ﷺ) as saying
For a faithful slave there are two rewards. By him in Whose hand is the life of Abu Huraira, but for Jihad in the cause of Allah, and Pilgrimage and kindness to my mother, I would have preferred to die as a slave. He (one of the narrators in the chain of transmitters) said: This news reached us that Abu Huraira did not perform Pilgrimage until his mother died for (keeping himself constantly) in her service.
This hadith has been transmitted on the authority of Abu Tahir but with a slight variation of words.
Abu Huraira reported Allah's Messenger (ﷺ) as saying
When a slave fulfils obligation of Allah and obligation of his master, he has two rewards for him. I narrated this to Ka'b, and Ka'b said: (Such a slave) has no accountability, nor has a poor believer.
The above hadith has been reported through another chain of transmitters on the authority of Abu Huraira.
Abu Huraira reported Allah's Messenger (ﷺ) as saying
It is good for a slave that he worships Allah well, and serves his master (well). It is good for him.