1 Corinthians 9:3
3 “My defence to them that examine me is this.” What is, “My defence to them that examine me is this?” “To those who seek to know whereby I am proved to be an Apostle, or who accuse me as receiving money, or inquire the cause of my not receiving, or would fain show that I am not an Apostle: to all such, my instruction given to you and these things which I am about to say, may stand for a full explanation and defence.” What then are these?
Source: Homilies on First Corinthians (New Advent)