Sahih Muslim Book 15, Hadith Number 4100.

Chapter : Reward of the slave when he is loyal to bit master and is good in worshipping Allah.

Abu Huraira reported Allah’s Messenger (may peace be upon him) 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.