1 Corinthians 9:16-18 New Revised Standard Version (NRSV)
16 If I proclaim the gospel, this gives me no ground for boasting, for an obligation is laid on me, and woe to me if I do not proclaim the gospel! 17 For if I do this of my own will, I have a reward; but if not of my own will, I am entrusted with a commission. 18 What then is my reward? Just this: that in my proclamation I may make the gospel free of charge, so as not to make full use of my rights in the gospel.