1 Samuel 24:3

Went in — To sleep there: Saul being a military man, used to sleep with his soldiers upon the ground. And it is not improbable, that being weary with his eager and almost incessant pursuit, first of David, then of the Philistines, and now of David again, he both needed and desired some sleep, God also disposing him thereto, that David might have this eminent occasion to demonstrate his integrity to Saul, and to all Israel. Of the cave — For that there were vast caves in those parts is affirmed, not only by Josephus, but also by Heathen authors; Strabo writes of one which could receive four thousand men.