Musnad Ahmad 30:2712

Hajjaj narrated to us, he said: Ibn Abi Dhi'b narrated to us, from Sa'id al-Maqburi, from his father, that he heard Abu Hurairah saying: "Were it not for two matters, I would have loved to be an owned slave; and that is because the owned slave cannot do anything with his wealth, and because I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: 'Allah did not create a slave who fulfills the right of Allah and the right of his master, except that Allah will pay him his reward twice.'"