Eloquence which diverts our minds to itself is harmful to its subject.
 
[L’eloquence faict injure aux choses, qui nous destourne à soy.]

Michel de Montaigne (1533-1592) French essayist

Essays, Book 1, ch. 26 “On the Education of Children [De l’institution des enfans]” (1579) (1.26) (1595) [tr. Ives (1925)]
    (Source)

First published in the 1580 edition.

(Source (French)). Alternate translations:

That eloquence offereth injurie unto things, which altogether drawes us to observe it.
[tr. Florio (1603), ch. 25]

That eloquence prejudices the subject it would advance, that wholly attracts us to itself.
[tr. Cotton (1686), ch. 25; Cotton/Hazlitt (1877)]

That sort of eloquence which makes us in love with Ourselves, does an injury to the subject it treats of.
[alt. tr. Cotton (1686), ch. 25]

The eloquence that diverts us to itself harms its content.
[tr. Frame (1943)]

When eloquence draws attention to itself it does wrong by the substance of things.
[tr. Screech (1987)]