1 Samuel 26:7 (KJV)

And David arose, and came to the place where Saul had pitched: and David beheld the place where Saul lay, and Abner the son of Ner, the captain of his host: and Saul lay in the trench, and the people pitched round about him.

Then answered David and said to Ahimelech the Hittite, and to Abishai the son of Zeruiah, brother to Joab, saying, Who will go down with me to Saul to the camp? And Abishai said, I will go down with thee.

So David and Abishai came to the people by night: and, behold, Saul lay sleeping within the trench, and his spear stuck in the ground at his bolster: but Abner and the people lay round about him.

Then said Abishai to David, God hath delivered thine enemy into thine hand this day: now therefore let me smite him, I pray thee, with the spear even to the earth at once, and I will not smite him the second time.

And David said to Abishai, Destroy him not: for who can stretch forth his hand against the Lord’s anointed, and be guiltless?