And Saul said to David, “You are not able to go against this Philistine to fight with him; for you are a youth, and he a man of war from his youth.”
But David said to Saul, “Your servant used to keep his father’s sheep, and when a lion or a bear came and took a lamb out of the flock, I went out after it and struck it, and delivered the lamb from its mouth; and when it arose against me, I caught it by its beard, and struck and killed it. Your servant has killed both lion and bear; and this uncircumcised Philistine will be like one of them, seeing he has defied the armies of the living God.” Moreover David said, “The Lord, who delivered me from the paw of the lion and from the paw of the bear, He will deliver me from the hand of this Philistine.”
And Saul said to David, “Go, and the Lord be with you!”
So Saul clothed David with his armor, and he put a bronze helmet on his head; he also clothed him with a coat of mail. David fastened his sword to his armor and tried to walk, for he had not tested them. And David said to Saul, “I cannot walk with these, for I have not tested them.” So David took them off.
Then he took his staff in his hand; and he chose for himself five smooth stones from the brook, and put them in a shepherd’s bag, in a pouch which he had, and his sling was in his hand. And he drew near to the Philistine. So the Philistine came, and began drawing near to David, and the man who bore the shield went before him. And when the Philistine looked about and saw David, he disdained him; for he was only a youth, ruddy and good-looking. So the Philistine said to David, “Am I a dog, that you come to me with sticks?” And the Philistine cursed David by his gods. And the Philistine said to David, “Come to me, and I will give your flesh to the birds of the air and the beasts of the field!”
Then David said to the Philistine, “You come to me with a sword, with a spear, and with a javelin. But I come to you in the name of the Lord of hosts, the God of the armies of Israel, whom you have defied. This day the Lord will deliver you into my hand, and I will strike you and take your head from you. And this day I will give the carcasses of the camp of the Philistines to the birds of the air and the wild beasts of the earth, that all the earth may know that there is a God in Israel. Then all this assembly shall know that the Lord does not save with sword and spear; for the battle is the Lord’s, and He will give you into our hands.”
So it was, when the Philistine arose and came and drew near to meet David, that David hurried and ran toward the army to meet the Philistine. Then David put his hand in his bag and took out a stone; and he slung it and struck the Philistine in his forehead, so that the stone sank into his forehead, and he fell on his face to the earth. So David prevailed over the Philistine with a sling and a stone, and struck the Philistine and killed him. But there was no sword in the hand of David. Therefore David ran and stood over the Philistine, took his sword and drew it out of its sheath and killed him, and cut off his head with it.
And when the Philistines saw that their champion was dead, they fled. Now the men of Israel and Judah arose and shouted, and pursued the Philistines as far as the entrance of the valley and to the gates of Ekron. And the wounded of the Philistines fell along the road to Shaaraim, even as far as Gath and Ekron. Then the children of Israel returned from chasing the Philistines, and they plundered their tents. And David took the head of the Philistine and brought it to Jerusalem, but he put his armor in his tent.
When Saul saw David going out against the Philistine, he said to Abner, the commander of the army, “Abner, whose son is this youth?”
And Abner said, “As your soul lives, O king, I do not know.”
So the king said, “Inquire whose son this young man is.”
Then, as David returned from the slaughter of the Philistine, Abner took him and brought him before Saul with the head of the Philistine in his hand. And Saul said to him, “Whose son are you, young man?”
So David answered, “I am the son of your servant Jesse the Bethlehemite.”
-1 Samuel 17:33-58