Got tipped off to this quote by reading Hal Draper. His translation is slightly different, and slightly more damning. But this version–from the Marxists Internet Archive–makes the point pretty well. It’s from a review Marx wrote in 1850. The whole review is well worth reading as a polemic against the mixed-up milieu of police spies, provocateurs, and professional insurrectionists.
“It is precisely their business to anticipate the process of revolutionary development, to bring it artificially to crisis-point, to launch a revolution on the spur of the moment, without the conditions for a revolution. For them the only condition for revolution is the adequate preparation of their conspiracy. They are the alchemists of the revolution and are characterised by exactly the same chaotic thinking and blinkered obsessions as the alchemists of old. They leap at inventions which are supposed to work revolutionary miracles: incendiary bombs, destructive devices of magic effect, revolts which are expected to be all the more miraculous and astonishing in effect as their basis is less rational. Occupied with such scheming, they have no other purpose than the most immediate one of overthrowing the existing government and have the profoundest contempt for the more theoretical enlightenment of the proletariat about their class interests.”