From wikiquote: 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).
Travels With Charley: In Search of America (1962) Pt. 3
Dictionary of Burning Words of Brilliant Writers (1895)
Laozi, Tao teh Ching, Ch. 78