Adversity makes men; prosperity makes monsters.

[L’adversité fait l’homme, et le bonheur les monstres.]

Proverbs, Sayings, and Adages

French proverb

Variants:

• "Adversity makes men, but prosperity makes monsters."
• "Adversity makes men, and prosperity makes monsters."
• "Prosperity makes monsters, but adversity makes men."

Often attributed to Victor Hugo, including from sources going back to the 19th Century (Ballou (1899)). I have not been able to find an actual citation or primary source.

It is also widely noted as an anonymous or proverbial saying (e.g., 1809, 1818).

It may well be a French proverb that was incorrectly attributed to Hugo (who wrote quite a bit on the subjects of adversity and prosperity) in order to have a name to hang off of it.