He may be credited with saying this; but not, it appears, by any reliable source.
Wikiquote, (when I checked), included it as one of two dozen quotes which it says are "Misattributed" to Mark Twain. It gives two sources that attribute it to the Reader's Digest, and says that it was "not found attributed to Twain until 2010". (Unfortunately the link for this attribution is dead.)
Even if the quote were verifiably attributed, a more generous question setter would surely give a stronger clue than "Which American". How about "Which American writer"?
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