1 Samuel 17:38-40

Then Saul gave David his own armor—a bronze helmet and a coat of mail. David put it on, strapped the sword over it, and took a step or two to see what it was like, for he had never worn such things before. “I can’t go in these,” he protested to Saul. “I’m not used to them.” So David took them off again. He picked up five smooth stones from a stream and put them into his shepherd’s bag. Then, armed only with his shepherd’s staff and sling, he started across the valley to fight the Philistine.