“There is a woman in every case; as soon as they bring me a report, I say, 'Look for the woman.”

Il y a une femme dans toutes les affaires; aussitôt qu'on me fait un rapport, je dis: «Cherchez la femme!»
[Dumas, Alexandre, Alexandre Dumas, père, Théâtre complet, http://www.archive.org/details/thtrecomplet24dumauoft, 2009-08-07, XXIV, 1889, Michel Lévy frères, éditeurs, Paris, French, 103], translation from The Penguin Dictionary of Quotations II.iii
See wikipedia cherchez la femme on how this phrase has come to be used.
Compare Juvenal satire VI.243 (circa 100 AD), "never yet was there a lawsuit which did not have a woman at the bottom of it" (translation by G. G. Ramsay), but in that case describing the litigiousness of Roman women.
Les Mohicans de Paris (The Mohicans of Paris) (1864 play)