1 Samuel 26:8,9

Now, therefore, let me smite him Although David would not kill him himself, when he had the like opportunity, 1 Samuel 24:4; yet Abishai thought he might give him leave to do it; and he pledged himself to nail him to the ground with his spear at one thrust, so that he should make no noise by crying out. And David said, Destroy him not Saul having been made king by God's special appointment, David looked upon it as a high crime to offer any violence to him: as if he had said, Though he be a tyrant, yet he is our lord and king; and I, although designed king, am yet his subject; and therefore I cannot kill him without sin, nor will I consent that thou shouldst do it.