Musnad Ahmad 30:2862

'Affan narrated to us, he said: Hammad narrated to us, from 'Ammar ibn Abi 'Ammar, he said: I heard Abu Hurairah saying: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "If the slave obeys his Lord and obeys his master, then he has two rewards."