1 Samuel 17:13
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 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.
The three eldest sons of Jesse who followed Saul to the battle were Eliab the firstborn, and next to him Abinadab, and the third Shammah. Their going took place during renewed war with the Philistines, when they joined Saul's army to fight the Philistines. Unlike his brothers David did not go to the battle at first but remained at home at his father's house shepherding the flock until he later came to the camp.
The verse belongs to the setting for David's later encounter with and victory over Goliath, the Philistine champion, while Israel's army was at war. That victory over Goliath later made David recognizable to the people of Achish and put him in peril when he sought refuge in Gath.