Behind every great fortune is/lies a crime.
NB: the original quote is actually "The secret of great fortunes without apparent cause is a crime forgotten, for it was properly done", but the shortcut version, although inaccurate, is a bit more catchy ;)
In French, the original version is: "Le secret des grandes fortunes sans cause apparente est un crime oublié, parce qu'il a été proprement fait" ; and I guess the short version would be like "Derrière chaque grande fortune se cache un crime". It comes from a dialog in the novel "Le Père Goriot".
Picture found on Wikiquote: https://en.wikiquote.org/wiki/File:HBalzac.jpg