1 Corinthians 9:7, 17

No soldier ever serves in the army and pays his own salary. No one ever plants a vineyard without eating some of the grapes. No person takes care of a flock without drinking some of the milk.

If I preach because it is my own choice, I have a reward. But if I preach and it is not my choice to do so, I am only doing the duty that was given to me.