1 Samuel 17:39-40

39David fastened Saul’s sword over his clothes and tried to walk, but he had never practiced doing this. “I can’t walk in these things,” David told Saul. “I’ve never had any practice doing this.” So David took all those things off.

40He took his stick with him, picked out five smooth stones from the riverbed, and put them in his shepherd’s bag. With a sling in his hand, he approached the Philistine.