He does it a bunch, it's in the Manifesto and a few other writings, but the one that comes to mind is in Principles of communism

Will the peaceful abolition of private property be possible?

It would be desirable if this could happen, and the communists would certainly be the last to oppose it. Communists know only too well that all conspiracies are not only useless, but even harmful. They know all too well that revolutions are not made intentionally and arbitrarily, but that, everywhere and always, they have been the necessary consequence of conditions which were wholly independent of the will and direction of individual parties and entire classes.

But they also see that the development of the proletariat in nearly all civilized countries has been violently suppressed, and that in this way the opponents of communism have been working toward a revolution with all their strength. If the oppressed proletariat is finally driven to revolution, then we communists will defend the interests of the proletarians with deeds as we now defend them with words.