From Preaching & Preachers, p. 107:
Indeed it seems to be the case that the greater the preacher the more hesitant he has been generally to preach. Oftentimes such men have had to be persuaded by ministers and elders and others to do this; they so shrank from the dread responsibility. This was true of George Whitefield, one of the greatest and most eloquent preachers ever to adorn a pulpit. And it has been true of many others. My argument is, therefore, that a man who feels that he is competent, and that he can do this easily, and so rushes to preach without any sense of fear or trembling, or any hesitation whatsoever, is a man who is proclaiming that he has never been ‘called’ to be a preacher. The man who is called by God is a man who realises what he is called to do, and he so realises the awefulness of the task that he shrinks from it. Nothing but this overwhelming sense of being called, and of compulsion, should ever lead anyone to preach.