While hiding in a cave, David secretly cut off a corner of whose robe?

Answer: Saul

In a cave near En-gedi David cut Saul's robe but refused to harm him, since he was the LORD's anointed.

“Soon Saul came to the sheepfolds along the road, where there was a cave, and he went in to relieve himself. And David and his men were hiding in the recesses of the cave.”— 1 Samuel 24:3 (BSB)