The oldest trace is a diary entry Benjamin Rush wrote on 22 September 1788, after calling on Benjamin Franklin: quacks prosper, Franklin argued, because patients talk up the remedies rather than admit their own credulity — “hence it was justly said, quacks were the greatest lyers in the world, except their patients.” That “it was justly said” is the useful part. Franklin was quoting, not coining, so the line is older than its most famous mouth and its author is gone.