We should keep silent about those in power; to speak well of them almost implies flattery; to speak ill of them while they are alive is dangerous, and when they are dead is cowardly.

[L’on doit se taire sur les puissants: il y a presque toujours de la flatterie à en dire du bien; il y a du péril à en dire du mal pendant qu’ils vivent, et de la lâcheté quand ils sont morts.]

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

The Characters [Les Caractères], ch. 9 “Of the Great [Des Grands],” § 56 (9.56) (1688) [tr. Stewart (1970)]
    (Source)

(Source (French)). Alternate translations:

The less we talk of the powerful, the better; what we say good of them, is often flattery: 'Tis dangerous to speak ill of 'em while they live, and villainous when they are dead.
[Bullord ed. (1696)]

The less we talk of the Great and Powerful, the better; what good we say of them is often Flattery 'Tis dangerous to speak ill of them while they are alive, and villainous when dead.
[Curll ed. (1713)]

The less we talk of the Great and Powerful, the better; what good we say of them is often Flattery: It is dangerous to speak of of them while living, it is base to insult over them when dead.
[Browne ed. (1752)]

The less we talk of the great and powerful the better; if we say any good of them, it is often almost flattery; it is dangerous to speak ill of them whilst they are alive, and cowardly when they are dead.
[tr. Van Laun (1885)]