I think it was Engels, and as I remember it, it wasn't a very elaborated reflection, rather dwarfed by his contrary views. I am sure that Marx would not think that you would have to make a fetish of revolution, and saw social change coming through all kinds of forces - war, legislation, famine for example. But both thought that capitalists had a material interest in the persistence of capitalism that they were unlikely to give up without some kind of struggle. Perhaps just as importantly, their practical experience of debate within the radical and working class movement was that those who sought a peaceful solution were actually looking for another goal altogether than that of socialism.