The quote is a paraphrase from a line in Honoré de Balzac's tragicomic
novel "Le Père Goriot." Here is the quote in its original French:
"Le secret des grandes fortunes sans cause apparente est un crime
oublié, parce qu' il a été proprement fait."
Institut de Linguistique Française: Balzac
An English translation:
"The secret of a great success for which you are at a loss to account
is a crime that has never been found out, because it was properly
Worldwide School: Father Goriot
Note that "grandes fortunes," translated above as "great success," can
also be translated as "great fortune(s)" or "great wealth." "Sans
cause apparente," translated above as "for which you are at a loss to
account," can be translated as "without apparent cause." "Un crime
oublié," translated above as "a crime that has never been found out,"
can be translated as "a forgotten crime" or "a lost crime." So there's
a lot of leeway here. The most common versions of the quote that I've
seen on the Web have shortened it somewhat, but the meaning is more or
less the same.
My strategy in locating this information was derived from my memory of
having read the quote many years ago. I thought I recalled the phrase
"crime oublié." The search string below led me to its source.
Google Web Search: "balzac" + "crime oublié"
If this is not the quote you're seeking, or if a link does not
function, please request clarification; I'll be glad to offer further
assistance before you rate my answer.