As favour and riches forsake a man, we discover in him the foolishness they concealed, and which no one perceived before.
 
[À mesure que la faveur et les grands biens se retirent d’un homme, ils laissent voir en lui le ridicule qu’ils couvraient, et qui y était sans que personne s’en aperçût.]

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

The Characters [Les Caractères], ch. 6 “Of Gifts of Fortune [Des Biens de Fortune],” § 4 (6.4) (1688) [tr. Van Laun (1885)]
    (Source)

(Source (French)). Alternate translations:

When Riches and Favour forsake a Man, we see presently he was a Fool, but no body could find it out in his Prosperity.
[Bullord ed. (1696)]

In proportion as Riches and Favour forsake a Man, we discover he was a Fool, which no body cou'd find out in his Prosperity.
[Curll ed. (1713)]

As Riches and Favour forsake a Man, we discover him to be a Fool, but no body could find it out in his Prosperity.
[Browne ed. (1752)]

As a man falls out of favour and his wealth declines, we discover for the first time the ridiculous aspects of his character, which were always there but which wealth and favour had concealed.
[tr. Stewart (1970)]