It takes a clever man to hide his cleverness.

[C’est une grande habileté que de savoir cacher son habileté.]

François VI, duc de La Rochefoucauld (1613-1680) French epigrammatist, memoirist, noble

Réflexions ou sentences et maximes morales [Reflections; or Sentences and Moral Maxims], ¶245 (1678) [tr. Heard (1917), ¶253]
    (Source)

In the 1665 edition, this read: Le plus grand art d’un habile homme est celui de savoir cacher son habileté.

(Source (French)). Alternate translations:

It is a Great Act of Wisdom to be able to Conceal one's being Wise.
[tr. Stanhope (1694), ¶246]

It requires no small degree of ability to know when to conceal it.
[pub. Donaldson (1783), "Ability," ¶4]

It is a great ability to be able to conceal one's ability.
[ed. Gowens (1851), ¶257]

There is great ability in knowing how to conceal one's ability.
[tr. Bund/Friswell (1871), ¶245]

It is the height of art to conceal art.
[tr. Stevens (1939), ¶245]

A very clever man will know how to hide his cleverness.
[tr. FitzGibbon (1957), ¶245]

It is exceedingly clever to know how to hide your cleverness.
[tr. Kronenberger (1959), ¶245]

To conceal ingenuity is ingenuity indeed.
[tr. Tancock (1959), ¶245]

It is great cleverness to know how to hide our cleverness.
[tr. Whichello (2016), ¶245]