1 Samuel 17:38-40

Then Saul dressed David in his garments and put a bronze helmet on his head, and put a coat of mail (armor) on him. Then David fastened his sword over his armor and tried to walk, [but he could not,] because he was not used to them. And David said to Saul, “I cannot go with these, because I am not used to them.” So David took them off. Then he took his [shepherd’s] staff in his hand and chose for himself five smooth stones out of the stream bed, and put them in his shepherd’s bag which he had, that is, in his shepherd’s pouch. With his sling in his hand, he approached the Philistine.