Source, in full
It seems likely that the attribution to Twain is apocryphal. It is not listed as authentic on Twainquotes, and is not listed at all in either R. Ken Ramussen's The Quotable Mark Twain (1998) or David W. Barber's Quotable Twain (2002)
More quotes by Mark Twain
Always do right. This will gratify some people, and astonish the rest.
You can't depend on your eyes when your imagination is out of focus.
Attribution
Quote text and notes from Wikiquote: Mark Twain (page history), available under CC BY-SA 4.0. Source quality and labels are our own summary of what Wikiquote records.