You injure yourself by your own vindictiveness, and give your enemies too great an advantage, if you accuse them of things that are not true, and tell lies in order to disparage them.

[C’est se venger contre soi-même, et donner un trop grand avantage à ses ennemis, que de leur imputer de choses qui ne sont pas vraies, et de mentir pour les décrier.]

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

The Characters [Les Caractères], ch. 11 “Of Mankind [De l’Homme],” § 150 (11.150) (1688) [tr. Stewart (1970)]
    (Source)

(Source (French)). Alternate translations:

We are reveng'd on our selves, and give our Enemies too much advantage over us, when we say things of them which are not true, and lie to reproach 'em.
[Bullord ed. (1696)]

'Tis reproaching our selves, and giving our Enemies too much advantage over us, to say things of them which are not true, and lie to disparage them.
[Curll ed. (1713)]

We are vindictive to our own loss, and give the Staff into our Enemies Hands, when we say things of them which are not true, and lie to defame them.
[Browne ed. (1752)]

To say things of our enemies which are not true, and to lie to defame them, is to avenge ourselves on ourselves, and give them too great an advantage over us.
[tr. Van Laun (1885)]