Musnad Ahmad 30:310

Abu Mu'awiyah narrated to us, al-A'mash narrated to us, from Abu Salih, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When the slave fulfills the right of Allah and the right of his masters, he has two rewards." He said: So I narrated them to Ka'b. Ka'b said: "There is no reckoning upon him, nor upon a believer who is abstinent."