Musnad Ahmad 30:2661

Yazid ibn Harun and Hashim narrated to us, they said: Ibn Abi Dhi'b informed us, from al-Maqburi, from Abu Hurayrah—Hashim said in his narration: from his father, that he heard Abu Hurayrah say: "Were it not for two matters, I would have loved to be a slave. That is because I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'Allah has not created a servant who fulfills the right of Allah and the right of his master, except that Allah will pay him his reward twice.'" Yazid said: "Indeed, the slave is not able to do anything with his wealth."