Sahih Muslim Book 27, Hadith 70
The above hadith has been reported through another chain of transmitters on the authority of Abu Huraira.
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.
It is good for a slave that he worships Allah well, and serves his master (well). It is good for him.