The attribution to Mark Twain is made at
http://www.quotelucy.com/keywords/conceals-quotes.html. The "quotelucy"
site gives no source in Mark Twain's work for the quotation, and the
site itself does not inspire confidence in the accuracy of the attribution.
For what it's worth, to me the quotation doesn't /sound/ at all like
Mark Twain. Any other votes on the question?