Who said this? Google comes up with numerous citations for both Mark Twain and Lincoln.

Unfortunately the quote is not in my Oxford Dictionary of Quotations.

Anyone have a definitive off-line or reputable on-line source?

This quote was not that long ago one of Anu's daily quotes. I remember it as being from Mark Twain.

Anyway Google: Twain + quotes, and a whole world turns up.

I'll give you one by Mark Twain back from my Dictionary of Cynical Quotations :

Golf is a good walk spoiled.

http://www.twainquotes.com/B.html

SILENCE
The unspoken word is capital. We can invest it or we can squander it.
- Mark Twain's Notebook

well , odds and ends

Regardless of the source, it was born in Washington DC (where not enough people remain silent).

For what it's worth, which may not be much, googling the phrase
remain silent and be thought a fool
with twain yields 775 hits but with lincoln, 22,200

This is weird to me. It's a very clever, quotable phrase. I don't understand why it is not in my (electronic) Oxford Dictionary of Quotations.

Just like circular definitions in a dictionary, this happens sometimes. In The Wordsworth Book of Literary Anecdotes, the bon mot: "He's a self made man who adores his maker" is credited to Disraeli, speaking of his aide (someone-or-other) and his aide, speaking of Disraeli.

Afterthought: Twain probably quoted Lincoln, and then got credit for it.

The number of quotes unverifiably attributed to Mark Twain, or to Lincoln, or Winston Churchill, or even Disraeli, is large. And I wouldn't want to guess at the multiple listings for the same quote, either.

"Those who cannot learn the lessons of history are doomed to repeat them."
-- Santayanna (or Lincoln, or Mark Twain, or...)

"Those who can learn the lessons of history are doomed to know that they are repeating them."

-- Faldage

Why don't we call it Anonymous and be done with?!

That guy, Anon, really had a lot to say!

You think Anon is prolific? You should read Ibid.

Makes you wonder if Op. Cit.'s attract. Well it may not make you wonder, but it made me wonder.

I prefer Op Cit. He can be repetative, but at least he skips around a bit.