These are probably not exactly what you want, but . . .

"It is better to remain silent and be thought a fool than to speak and remove all doubt." Benjamin Franklin, I think

"Nothing is so firmly believed as that which we least know." Michel de Montaigne

"Wise men talk because they have something to say; fools, because they have to say something." - Plato

I'm using the last one as my senior quote for the yearbook.