1 Corinthians 9:1,13-16

1Am I not an apostle? am I not free? have I not seen Jesus Christ our Lord? are not ye my work in the Lord? ... 13Do ye not know that they which minister about holy things live of the things of the temple? and they which wait at the altar are partakers with the altar? 14Even so hath the Lord ordained that they which preach the gospel should live of the gospel. 15But I have used none of these things: neither have I written these things, that it should be so done unto me: for it were better for me to die, than that any man should make my glorying void. 16For though I preach the gospel, I have nothing to glory of: for necessity is laid upon me; yea, woe is unto me, if I preach not the gospel!

Comment: Paul asserts that he has seen the resurrected Jesus with his own eyes. In fact, even though he has a right as a preacher to be paid, he refuses that right. This refusal shows he had nothing to gain from preaching the resurrected Jesus. Refusing payment is evidence of his deep sincerity.