General Discussion

There is substantial evidence that this was said by David Hannum , a Barnum competitor, who intended to disparage Barnum & his audiences after a dispute over the Cardiff Giant hoax. When Hannum wouldn't see the giant to Barnum, he simply invented his own "mummified" hoax. Hannum, in an effort to denigrate the competition used this phrase which is odd since he was already promoting his own hoax.
There is some history to suggest thr phrase originally was publicly used by Joespeh Bessimer to explain his penchant for running scams & cons.
But, as Barnum was known to respect his customers, it's very unlikely he ever said such a thing, and there is no such reference in his personal writings nor any direct quote from contemporary reporters.
Quoting him as saying that is a black eye that Barnum doesn't really deserve.