1 Samuel 17:20

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.

Cross-reference

1 Samuel 17:28 shows Eliab's angry reaction to David's arrival at the battle line, described in verse 20.

1 Samuel 26:5 describes David approaching Saul's camp — similar to his approach to the battle camp in verse 20.