1 Corinthians 9:16-18

For if I preach the gospel, I have nothing to boast of, for I am under compulsion; for woe is me if I do not preach the gospel. For if I do this voluntarily, I have a reward; but if against my will, I have a stewardship entrusted to me. What then is my reward? That, when I preach the gospel, I may offer the gospel without charge, so as not to make full use of my right in the gospel.