1 Samuel 26:6-9

6David spoke to two of his men. One of them was Ahimelech, a Hittite man. The other man was Abishai, who was Joab's brother (Zeruiah was their mother). David asked them, ‘Which one of you will go down into Saul's camp with me?’ Abishai said, ‘I will come with you.’

7So David and Abishai went among Saul's army that night. They saw that Saul was asleep in the middle of the camp. He had pushed his spear into the ground near his head. Abner and the army were asleep around Saul.

8Abishai said to David, ‘Tonight God has given your enemy into your power. I will push Saul's spear through his body and fix him to the ground. I will only have to do it once to kill him. I will not need to do it again.’

9But David said to Abishai, ‘Do not kill Saul. He is the man that the Lord has chosen to be Israel's king. Anyone who attacks him will be guilty of a sin.