True eloquence consists in saying all that need be said and no more.

[La véritable éloquence consiste à dire tout ce qu’il faut, et à ne dire que ce qu’il faut.]

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

Réflexions ou sentences et maximes morales [Reflections; or Sentences and Moral Maxims], ¶250 (1665-1678) [tr. FitzGibbon (1957)]
    (Source)

Present in the 1st (1665) edition. In manuscript it begins "L’éloquence est de ne dire que ce qu’il faut ..."

(Source (French)). Other translations:

True Eloquence consists in saying whatever is requisite, and in not saying any more then what is requisite.
[tr. Davies (1669), ¶45]

True Eloquence consists in Saying all that is Fit to be Said; and Leaving Out all that is not Fit to be Said.
[tr. Stanhope (1694), ¶251]

True eloquence consists in saying all that is proper, and nothing more.
[pub. Donaldson (1783), ¶110]; ed. Lepoittevin-Lacroix (1797), ¶236]

True eloquence consists in saying what is proper, but nothing more.
[ed. Carvill (1835), ¶97]

True eloquence consists in saying all that is necessary, and nothing but what is necessary.
[ed. Gowens (1851), ¶262]

True eloquence consists in saying all that should be, not all that could be said.
[tr. Bund/Friswell (1871), ¶250]

True eloquence lies in saying everything one should say, but nothing that one should not.
[tr. Heard (1917), ¶258]

True eloquence consists in saying the right thing, and nothing more.
[tr. Stevens (1939), ¶250]

True eloquence means saying all that is necessary and only what is necessary.
[tr. Kronenberger (1959), ¶250]

True eloquence consists in saying all that is required and only what is required.
[tr. Tancock (1959), ¶250]

True eloquence consists in saying, on the one hand all that we ought to say, on the other omitting what we ought not.
[tr. Whichello (2016) ¶250]