I have often seen the phrase "anarchy is order" attributed to Proudhon—and to Bakunin, and Bellarrigue, and Elisee Reclus, and a French singer-songwriter named Leo Ferre. Often the phrase is actually Bellegarrigue's ("Anarchy is order; government is civil war") or the phrase "Anarchy is order without power," cited as appearing in the Confessions of a Revolutionary. That latter phrase does not seem to appear in that book (and I've searched pretty carefully) and it doesn't really sound all that much like Proudhon. There are a number of places where he talked about the relationship between anarchy and order, and lots of places where he talked about the fact that liberty is the principle (or "mother," in the famous phrase) of order. Curiously, though, the closest I could come to the actual phrase so often cited was this passage from The General Idea of the Revolution:
The passage has been translated as:Croit-on qu'on lieu de rétablir les justices seigneuriales et les parlements sous d'autres noms et d'autres formes, de refaire l'absolutisme en le baptisant du nom de Constitution, d'esservir les provinces comme auparavant, sous prétexte d'unité et de centralisation; de sacrifier de nouveau toutes les libertés, en leur donnant pour compagnon inséparable un prétendu ordre public, qui n'est qu'anarchie, corruption et force brutale; croit-on, dis-je, qu'ils n'eussent acclamé le nouveau régime, achevé la révolution, si leur regard avait pénétré dans cet organisme que leur instinct cherchait, mais que l'état des connaissances et les préoccupations du moment ne leur permettaient pas de deviner?...
Can it be believed that, instead of reestablishing the seignorial courts and the parliaments under other names and other forms, of re-erecting abolutism after baptising it with the name of the Constitution, of enslaving the provinces as before, under the pretext of unity and centralization, of sacrificing all liberties, by giving them for an inseparable companion a pretended public order, which is but confusion, corruption and brute force—can it be believed, I say, that they would not have welcomed the new order, and completed the Revolution, if their sight had penetrated the organism which their instinct sought, but the state of knowledge and the distractions of the moment did not permit them to conceive?The French reminds us that even as late as 1851, Proudhon often still used the word "anarchie" to describe disorder, so here we have a claim that "order... is only anarchy," but it is a "so-called public order" which is "only anarchy, corruption and brutal force." John Beverley Robinson, in his translation, chose to render "anarchie" as "confusion" in this case, and the title of the section in which the passage appears, "Anarchie des forces économiques," as "Chaos of Economic Forces."
Robinson may have rendered a service at the time, but it's one of a number of similar decisions that probably trip up us a bit now, when arguably it would be nice not to be shielded from all the tensions in Proudhon's work. I think that The General Idea, which is, I think, generally considered one of Proudhon's least controversial works, but which comes from a period where he was certainly not averse to bold, complex statements (such as the infamous The Revolution as Demonstrated by the Coup d'Etat of December 2), might read rather differently as an anarchist text in which "anarchy" as often as not means disorder.
Unpacking this sort of potential contradiction in arguably "foundational" texts is, of course, a sort of high-risk enterprise, for a variety of reasons. But my sense is that we have every opportunity to gain from the encounter. Our current sense of the significance of these texts, however well or ill-founded is already a string to our bow. Nothing says that the understanding we have built is worthless, even if it turns out it wasn't quite what Proudhon had in mind. And nothing commits us to whatever else we find in a rereading and rethinking. I think it is probably inevitable that our readings of historical texts will tend to have a double character anyway, with a present-oriented interpretation working alongside whatever we are able to glean about the contexts of composition and original composition. A text like The General Idea is fairly comfortably ensconced in the anarchist literature at this point, despite the many strange elements it contains. Perhaps our understanding would be opened up by treating it as it appears to have been originally presented: a work in which two visions of "anarchy" must almost certainly have been in play. I wouldn't be surprised if there was another antinomy—another of those productive contradictions Proudhon was so fond of—to be grasped in the play of anarchies and order.