Where exactly does Karl Marx or Friedrich Engels describe the eventual demise (or implosion?) of capitalism?

The inevitability of capitalism's destruction is prevalent in much of Marx's writing.  However, its direct mention can be seen in the opening chapter of Marx's and Engels's The Communist Manifesto.
An essential component of Marx's thesis is the inevitability or dialectical materialism.  He argues that history progresses in this unstoppable unfolding that allows the conditions for change to take place. Essentially, this means, that capitalism will eventually disappear.  Marx opens The Communist Manifesto with such an idea.  He believes that capitalism is progressing with so much speed that a breakdown is bound to happen:

Because there is too much civilisation, too much means of subsistence, too much industry, too much commerce. The productive forces at the disposal of society no longer tend to further the development of the conditions of bourgeois property; on the contrary, they have become too powerful for these conditions, by which they are fettered, and so soon as they overcome these fetters, they bring disorder into the whole of bourgeois society, endanger the existence of bourgeois property. The conditions of bourgeois society are too narrow to comprise the wealth created by them.

Marx sees capitalism as a "runaway train."  The industrialists' desire for wealth has created a world of "too much industry."  Marx sees this unfettered desire for wealth as unsustainable.  Eventually, a world of "too much commerce" will run out, creating the conditions for its eventual destruction.
At the same time, Marx sees capitalism's destruction in the consolidation of the people who suffer the most under it.  Marx believes that for every industrialist benefitting from capitalism, there are many more who do not.  When these individuals unify, Marx sees the implosion of capitalism as inevitable:

The lower strata of the middle class — the small tradespeople, shopkeepers, and retired tradesmen generally, the handicraftsmen and peasants — all these sink gradually into the proletariat, partly because their diminutive capital does not suffice for the scale on which Modern Industry is carried on, and is swamped in the competition with the large capitalists, partly because their specialised skill is rendered worthless by new methods of production. Thus the proletariat is recruited from all classes of the population.

The recruitment of the proletariat from "all classes of the population" is why Marx feels that capitalism is doomed. He believes that once all of those toiling under capitalism realize they are not alone and pull together as one, they will be an unstoppable force.  Marx goes on to explain that "what the bourgeoisie therefore produces, above all, are its own grave-diggers."  Each success the wealthy experience means hardship for more people.  Marx believes that the proletariat will not suffer in silence, but rather consolidate as one entity.  They will be united in their shared disdain for capitalism and resoundingly call for change.  This is the reason why capitalism's "fall and the victory of the proletariat are equally inevitable."