1 Samuel 17:39-40

39David strapped his sword on over the military clothes and tried to walk, but he was not used to them. “I can’t walk in these,” David said to Saul, “I’m not used to them.” So David took them off.

40Instead, he took his staff in his hand and chose five smooth stones from the wadi and put them in the pouch, in his shepherd’s bag. Then, with his sling in his hand, he approached the Philistine.