1 Samuel 17:18 (KJV)

And the three eldest sons of Jesse went and followed Saul to the battle: and the names of his three sons that went to the battle were Eliab the firstborn, and next unto him Abinadab, and the third Shammah.

And David was the youngest: and the three eldest followed Saul.

But David went and returned from Saul to feed his father's sheep at Bethlehem.

And the Philistine drew near morning and evening, and presented himself forty days.

And Jesse said unto David his son, Take now for thy brethren an ephah of this parched corn, and these ten loaves, and run to the camp of thy brethren;

Now Saul, and they, and all the men of Israel, were in the valley of Elah, fighting with the Philistines.

And David rose up early in the morning, and left the sheep with a keeper, and took, and went, as Jesse had commanded him; and he came to the trench, as the host was going forth to the fight, and shouted for the battle.

For Israel and the Philistines had put the battle in array, army against army.

And David left his carriage in the hand of the keeper of the carriage, and ran into the army, and came and saluted his brethren.

And as he talked with them, behold, there came up the champion, the Philistine of Gath, Goliath by name, out of the armies of the Philistines, and spake according to the same words: and David heard them.