Saul came to some sheep pens beside the road. There was a cave near there, so Saul went in to relieve himself. David and his men were deep inside that same cave.
David's men told him, "This is day the Lord told you about when he said, 'I will give your enemy to you, and you can do whatever you want to him.'" So David crawled closer and closer to Saul and cut off a corner of Saul's robe. Saul didn't notice what happened.
He said to his men, "I pray the Lord never lets me do anything like that to my master again. I must not do anything against Saul, because he is the Lord's chosen king. "
Later, David came out of the cave and called out to Saul, "My lord the king!" Saul looked back. David bowed with his face to the ground to show his respect.
You can see that with your own eyes that is not true. The Lord put you within my grasp today in the cave, but I refused to kill you. I was merciful to you. I said, 'I won't hurt my master. Saul is the Lord's chosen king.'
Look at this piece of cloth in my hand. I cut off the corner of your robe. I could have killed you, but I didn't. Now, I want you to understand this. I want you to know that I am not planning anything against you. I did nothing wrong to you, but you are hunting me and trying to kill me.
This shows that you are not my enemy—A man doesn't catch his enemy, and then just let him go. He doesn't do good things for his enemy. May the Lord reward you for being good to me today.
Now make a promise to me. Use the Lord's name and promise that you will not kill my descendants, even after I die. Promise me that you will not erase my name from my father's family."
So David made a promise to Saul. He promised that he would not kill Saul's family. Then Saul went back home. David and his men went back up to the fort.