Midrasch zu Wajikra 25:43

2) (Vayikra 25:43) ("You shall not rule over him oppressively, and you shall fear your G d.") "You shall not rule over him oppressively": Do not tell him "Heat up this cup for me" if you do not need it, "Cool this cup for me," if you do not need it, "Dig under this grape-vine until I come." And lest you say "I really do need it," such things are relegated to the heart, (i.e., to the truth that one knows in his heart). And of all things that are relegated to the heart it is written "and you shall fear your G d," (who probes the heart).