Saul listened to Jonathan's advice and swore an oath: "As surely as the Lord lives, David will not be killed."
1 Samuel 19:6 In-Context
Jonathan spoke well of David to Saul his father and said to him, “Let not the king do wrong to his servant David; he has not wronged you, and what he has done has benefited you greatly.
He took his life in his hands when he killed the Philistine. The LORD won a great victory for all Israel, and you saw it and were glad. Why then would you do wrong to an innocent man like David by killing him for no reason?”
Saul listened to Jonathan and took this oath: “As surely as the LORD lives, David will not be put to death.”
So Jonathan called David and told him the whole conversation. He brought him to Saul, and David was with Saul as before.
Once more war broke out, and David went out and fought the Philistines. He struck them with such force that they fled before him.