Musnad Ahmad 30:1948

Aswad ibn Amir narrated to us, Isra'il narrated to us, from al-A'mash, from Abu Salih, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "How excellent is the slave if he fulfills the right of Allah and the right of his masters." Ka'b said: Allah and His Messenger have spoken the truth; there is no reckoning upon him, nor upon any ascetic believer.