No man is exempt from saying silly things; the mischief is to say them deliberately.

[Personne n’est exempt de dire des fadaises: le malheur est, de les dire curieusement]

Michel de Montaigne (1533-1592) French essayist

Essays, Book 3, ch. 1 “Of the Useful and the Honorable [De l’utile et de l’honnête]” (1586) (3.1) (1595)
    (Source)

First appeared in the 2nd (1588) edition. (Source (French)). Alternate translations:

No man living is free from speaking foolish things; the ill lucke is, to speake them curiouslie.
[tr. Florio (1603)]

No Man is free from speaking foolish things; but the worst on't is when a Man studies to play the Fool.
[tr. Cotton (1686)]

No man is free from speaking foolish things; but the worst on it is, when a man labors to play the fool.
[tr. Cotton/Hazlitt (1877)]

No one is exempt from saying foolish things; the misfortune is to say them intentionally.
[tr. Ives (1925)]

No one is exempt from saying silly things. The misfortune is to say them with earnest effort.
[tr. Frame (1943)]

No one is free from uttering stupidities. The harm lies in doing it meticulously.
[tr. Screech (1987)]