1 Samuel 17:38-40

Then Saul clothed David with his armor. He put a helmet of bronze on his head and clothed him with a coat of mail, and David strapped his sword over his armor. And he tried in vain to go, for he had not tested them. Then David said to Saul, “I cannot go with these, for I have not tested them.” So David put them off. Then he took his staff in his hand and chose five smooth stones from the brook and put them in his shepherd’s pouch. His sling was in his hand, and he approached the Philistine.