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)

“How strange," continued the king, with some asperity; "the police think that they have disposed of the whole matter when they say, 'A murder has been committed,' and especially so when they can add, 'And we are on the track of the guilty persons.'”

Alexandre Dumas book The Count of Monte Cristo

Chapter 11 http://en.wikisource.org/wiki/The_Count_of_Monte_Cristo/Chapter_11. The Count of Monte Cristo (1845–1846)

“There is … a clever maxim which bears upon what I was saying to you some little while ago, and that is, that unless wicked ideas take root in a naturally depraved mind, human nature, in a right] and wholesome state, revolts at [[crime. Still, from an artificial civilisation have originated wants, vices, and false tastes, which occasionally become so powerful as to stifle within us all good feelings, and ultimately to lead us into guilt and wickedness…”

Alexandre Dumas book The Count of Monte Cristo

Chapter 17 http://en.wikisource.org/wiki/The_Count_of_Monte_Cristo/Chapter_17 The Count of Monte Cristo (1845–1846)

“My friend, the pleasures to which we are not accustomed oppress us more than the griefs with which we are familiar.”

Alexandre Dumas

Le Vicomte de Bragelonne ou Dix ans plus (The Vicomte de Bragelonne) (1847)

“You are young, and your bitter recollections have time to change themselves into sweet remembrances.”

Alexandre Dumas book The Three Musketeers

Source: The Three Musketeers (1844), Ch. 67: Conclusion.

“We are never quits with those who oblige us," was Dantes' reply; "for when we do not owe them money, we owe them gratitude.”

Alexandre Dumas book The Count of Monte Cristo

Chapter 2 http://en.wikisource.org/wiki/The_Count_of_Monte_Cristo/Chapter_2 The Count of Monte Cristo (1845–1846)

“The chains of wedlock are so heavy that it takes two to carry them; sometimes three.”

Alexandre Dumas

Les chaînes du mariage sont si lourdes qu'il faut être deux pour les porter; quelquefois trois. Attributed to Dumas in: Elizabeth Abbott, Une histoire des maîtresses http://books.google.gr/books?id=fEsPUICzDY4C&dq=, Les Éditions Fides, 2004, p. 16. Attributed

“Nothing succeeds like success.”

Alexandre Dumas book Ange Pitou

Rien ne réussit comme le succès. Ange Pitou, Vol. 1 chapter 7 http://www.dumaspere.com/pages/biblio/chapitre.php?lid=r3&cid=7 (1854).

“Private misfortunes must never induce us to neglect public affairs.”

Alexandre Dumas book The Count of Monte Cristo

chapter 5 http://en.wikisource.org/wiki/The_Count_of_Monte_Cristo/Chapter_5 The Count of Monte Cristo (1845–1846)