The phrase "la propriété, c'est la liberté" appears in the Memoirs of a Revolutionist (1867, Lacroix, p.128) and in the posthumous Théorie de la propriété, (1867, Lacroix, p. 183) where Proudhon writes as if he had said that in 1846, in the System of Economic Contradictions. What he says in the 1846 is actually this, as far as I can tell:
"Property, with regard to facts and to rights, is essentially contradictory, and it is for this reason that it is anything at all. Indeed, Property is the right of occupation, and at the same time the right of exclusion. Property is the reward of labor, and the negation of labor. Property is the spontaneous product of society, and the dissolution of society. Property is an institution of justice, and property IS THEFT."
And from the "Revolutionary Program" of 1848:
I am, as you are well aware, citizens, the man who wrote these words: Property is theft!
I do not come to retract them, heaven forbid! I persist in regarding this provocative definition as the greatest truth of the century. I have no desire to insult your convictions either: all that I ask, is to say to you how, partisan of the family and of the household, adversary of communism, I understand that the negation of property is necessary for the abolition of misery, for the emancipation of the proletariat. It is by its fruits that one must judge a doctrine: judge then my theory by my practice.
When I say, Property is theft! I do not propose a principle; I do nothing but express one conclusion. You will understand the enormous difference presently.
However, if the definition of property which I state is only the conclusion, or rather the general formula of the economic system, what is the principle of that system, what is its practice, and what are its forms?
My principle, which will appear astonishing to you, citizens, my principle is yours; it is property itself.
I have no other symbol, no other principle than those of the Declaration of the Rights of Man and of the Citizen: Liberty, equality, security, property.
Like the Declaration of Rights, I define liberty as the right to do anything that does not harm others.
Again, like the Declaration of Rights, I define property, provisionally, as the right to dispose freely of one's income, the fruits of one's labor and industry.
Here is the entirety of my system: liberty of conscience, liberty of the press, liberty of labor, free trade, liberty in education, free competition, free disposition of the fruits of labor and industry, liberty ad infinitum, absolute liberty, liberty for all and always?
It is the system of '89 and '93; the system of Quesnay, of Turgot, of J.-B. Say; the system that is always professed, with more or less intelligence and good faith, by the various organs of the political parties, the system of the Débats, of the Presse, of the Constitutionnel, of the Siècle, of the Nationale, of the Rèforme, of the Gazette; in the end it is your system, voters.
Simple as unity, vast as infinity, this system serves for itself and for others as a criterion. In a word it is understood and compels adhesion; nobody wants a system in which liberty is the least bit undermined. One word identifies and wards off all errors: what could be easier than to say what is or is not liberty? Liberty then, nothing more, nothing less. Laissez faire, laissez passer, in the broadest and most literal sense; consequently property, as it rises legitimately from this freedom, is my principle. No other solidarity between the citizens than that accidents resulting from chance. . .
More coming soon.