1 Samuel 17:15 (KJV)

And the Philistine said, I defy the armies of Israel this day; give me a man, that we may fight together.

When Saul and all Israel heard those words of the Philistine, they were dismayed, and greatly afraid.

Now David was the son of that Ephrathite of Bethlehemjudah, whose name was Jesse; and he had eight sons: and the man went among men for an old man in the days of Saul.

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.

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;

And carry these ten cheeses unto the captain of their thousand, and look how thy brethren fare, and take their pledge.

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.