Musnad Ahmad 30:2601

Waki' 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: "Let not one of you say to his slave: 'My slave' (abdi), but let him say: 'My young man' (fatay). And let not the slave say to his master: 'My lord' (rabbi), but let him say: 'My master' (sayyidi)."