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We seldom regret talking too little, but very often talking too much. This is a well-known maxim which everybody knows and nobody practices.

[L’on se repent rarement de parler peu, très souvent de trop parler: maxime usée et triviale que tout le monde sait, et que tout le monde ne pratique pas.]

Jean de La Bruyère (1645-1696) French essayist, moralist

The Characters [Les Caractères], ch. 11 “Of Mankind [De l’Homme],” § 149 (11.149) (1688) [tr. Van Laun (1885)]
    (Source)

(Source (French)). Alternate translations:

We seldom repent talking too little, but very often talking too much, a common and trivial maxim which every body knows, and no body practices.
[Bullord ed. (1696) and Curll ed. (1713)]

We seldom repent talking too little, but very often talking too much; a common obsolete Maxim, which every body knows, and no body practices.
[Browne ed. (1752)]

We seldom repent of speaking little, and very often of speaking too much; a well-worn and familiar maxim, that everyone knows but that not everyone practices.
[tr. Stewart (1970)]