I scourged the net for a possible credible source linking this phrase to Mark Twain but I couldn't find it. It's more likely that someone intentionally attributed the quote to Mark Twain to make it sound more credible (After all Mark Twain was known for his witty quotes). Subsequently, the quote might have spread on the internet through memes and it became Mark Twain's quote in the eyes ************SPAM/BANNEAR************ the internet users.
Thanks for the copy paste from another website. You might be right, however. I always attributed that quote to Mark Twain like many other witty things he said but now that I read your copy paste, I begin to wonder if there is a reliable source to prove it. It is, nevertheless, completely irrelevant whom actually did say it in the first place, be it Samuel Clemens or not, ************SPAM/BANNEAR************ course.