According to most discussions of Euclid's Elements, this work - and indeed, much of Ancient Greek geometry - should be seen as engaged in the game of figuring out what can be done with straightedge and (collapsible) compass.
Although Euclid's Elements can certainly be (mathematically) interpreted in this way, Euclid never says anything to explicitly support this. Nor does one find an explicit discussion of the idea that this is how we should view the ancient practice of geometry (with respect to Euclid or the Greek geometrical tradition in general) in Plato, Aristotle, Proclus and so on - as far as I can tell.
Is there any ancient source who specifically talks about straightedges and compasses and the idea that much of geometry revolves around questions of what can and can't be done with such tools? I am aware that much discussion of Greek mathematics simply assumes this, so I'm not looking for references to modern people who repeat this piece of folklore - there are examples of that everywhere. I'm rather looking for actual extant ancient texts that support this piece of folklore.