I Samuel 17:21-23, 32-33
Israel and the Philistines drew up for battle, army against army. David left the things in charge of the keeper of the baggage, ran to the ranks, and went and greeted his brothers. As he talked with them, the champion, the Philistine of Gath, Goliath by name, came up out of the ranks of the Philistines, and spoke the same words as before. And David heard him.
David said to Saul, "Let no one's heart fail because of him; your servant will go and fight with this Philistine." Saul said to David, "You are not able to go against this Philistine to fight with him; for you are just a boy, and he has been a warrior from his youth."