I have always observed that to succeed in the world one should appear like a fool but be wise.

[J’ai toujours vu que, pour réussir parfaitement bien dans le monde, il alloit avoir l’air fou et être sage.]

Charles-Lewis de Secondat, Baron de Montesquieu (1689-1755) French political philosopher

Pensées Diverses [Assorted Thoughts], # 1004 / 1013 (1720-1755)

This a common English translation since at least 1896 (original source unknown).

(Source (French)). Other translations:

To succeed perfectly well in the world, I have always seen that you have to appear mad while being wise.
[tr. Clark (2012)]