Musnad Ahmad 30:1078

The Messenger of Allah (peace and blessings of Allah be upon him) said: "Let not one of you say: 'Feed your lord, give your lord to drink, perform ablution for your lord.' And let not one of you say: 'My lord.' Rather, let him say: 'My master and my patron.' And let not one of you say: 'My male slave' or 'My female slave.' Rather, let him say: 'My young man, my young woman, and my servant.'"