From Samuel Orchart Beeton's Book of Jokes and Jests:

Dionysius, the tyrant of Syracuse, was tormented by the conspiracies incessantly formed against his throne and person. One day a man presented himself at a public levee, and told the monarch that he knew of a means by which the monarch might discover any conspiracy against him, and that for a certain sum of money he would reveal it to him. Dionysius promised to give him what he asked, upon which the man, taking him aside, said to him, "I possess no such secret, but if you tell your subjects that I have revealed to you one that is infallible, no one will henceforth dare to conspire against you." Dionysius thought the advice excellent, adopted it, paid the money, and lived tranquilly thereafter.