מדרש אויף ישעיהו 55:4

Many will see and be afraid. Whoever wants to do repentance should look at David. And so it says (Isaiah 55:4) "I have given him as a witness to peoples." Therefore many will see and be afraid. And so it says (Exodus 14:31) "And Israel saw the great hand that the Lord had inflicted upon the Egyptians." Immediately then, Moses sang....

"I have sinned against you alone, and done what is evil in your sight, so that you may be justified in your words and blameless in your judgment. Why was David similar to this? To one who is broken and comes to the doctor, and the doctor is amazed and says to him, 'How great is your wound, I am so sorry for you.' The one who is broken replies, 'You are only sorry for me, not for yourself. I did not break for my own sake, but for your reward.' So too, the Holy One, blessed be He, said to Himself, 'I have sinned against You alone.' When you say to sinners, 'Why didn't you repent?' if you accept me, all sinners will complete their repentance and look to me. And I will be a witness and testify to the entire world that you accept those who repent. And so He says to him (Isaiah 55:4), 'I made him a witness to the peoples.' Not just me, but all of Israel. And He also says (Isaiah 43:10), 'You are my witnesses, declares the Lord, and my servant whom I have chosen.'"