Source, in full
The first appearance of this quote in print was in the July 1908 issue of the journal Profitable Advertising under the heading "Modernized Maxims." It next appeared in the June 1911 issue of The Philistine where Elbert Hubbard labeled it: "motto for a hotel-keeper." In the 1920s, it was published with the label: "Pickpocket's motto." The attribution to P.T. Barnum didn't appear in print until a 1934 article in Reader's Digest.