The secret of grand fortunes without apparent cause is a crime forgotten, for it was properly done.
[Le secret des grandes fortunes sans cause apparente est un crime oublié, parce qu’il a été proprement fait.]
Honoré de Balzac (1799-1850) French novelist, playwright
Le Père Goriot, Part 2 (1835) [tr. Marriage]
Often paraphrased as "Behind every great fortune there is a great crime."