I Samuel - Chapter 17
David and Goliath
The Philistines set up a camp on the opposite side of a valley from the Tribe of Judah. They had a champion named Goliath who was a giant of 12 or 13 feet tall. (That may not be as supernaturally large as some people like to depict him, but you have to admit that it's pretty intimidating!) He was so big that his spear was the size of a weaver's beam and the blade alone weighed 25-30 pounds. Every day, morning and evening, Goliath would come and taunt the Jews, challenging them to send a champion to face him in single combat. (No takers.)
David's three oldest brothers were in Shaul's army, in the camp facing Goliath. While he was home from his harp-playing duties, Jesse sent David to bring supplies to his brothers. While he was visiting the camp, David saw the incident with Goliath and asked what was going on. A soldier told him that Shaul had promised riches and his daughter to the man who kills Goliath. David was merely unimpressed that this crass Philistine should disgrace the Jewish army.
At this point, David's brother Eliav came by and yelled at him for neglecting his sheep-tending. Unaware that their father had sent him, he accused David of coming to watch the fighting. It could be that Eliav knew what David was planning and was trying to discourage him from taking unnecessary risks. In any case, Eliav knew that David had been anointed and it was inappropriate to talk to David this way.
Soldiers had overheard David's words and repeated them to Shaul. Shaul summoned David, who said that no one should lose heart because of Goliath; he, David, would kill him himself! Shaul pointed out the obvious: David was no soldier. What he didn't know was that, as a shepherd, G-d had sent a lion and a bear to attack David's flocks, so that David would know what he was capable of. Shaul agreed to let David try. He tried to clothe David in his own armor, but David declined, being unused to maneuvering in armor. (The Targum Yonasan says that David wanted the miracle of Goliath's defeat to be even greater, which is why he delined armor.) David took only his staff, his sling and five smooth stones.
Goliath saw the shepherd boy approach with his staff and laughed. "Are you going to beat me like a dog?" David replied that swords and spears don't win battles: G-d does. David lobbed a stone at Goliath, which penetrated his forehead. The giant keeled over, dead.
David took Goliath's sword and used it to cut off the giant's head. Terrified, the Philistine army scattered, the army of Israel in hot pursuit.
Amazed by this stunning victory, Shaul asked his general Avner whose son David was. Of course, he knew that Jesse was David's father; by this he meant that he wanted to know more about David's background - a history of military valor, worthiness to marry his daughter, and the possibility that David was the usurper that Shaul feared.