I was with the Prophet (ﷺ) when a man who was a murderer and had a strap round his neck was brought to him.
He then called the legal guardian of the victim and asked him: Do you forgive him?
He said: No. He asked: Will you accept the blood-money? He said: No. He asked: Will you kill him? He said: Yes. He said: Take him. When he turned his back, he said: Do you forgive him? He said: No. He said: Will you accept the blood-money? He said: No. He said: Will you kill him? He said: Yes. He said: Take him. After repeating all this a fourth time, he said: If you forgive him, he will bear the burden of his own sin and the sin of the victim. He then forgave him. He (the narrator) said: I saw him pulling the strap.