An Nasai: Hadith 4740

"I saw the Messenger of Allah when the heir of a victim brought the killer, leading him by a string. The Messenger of Allah said to the heir of the victim: Will you forgive him?' He said: 'No., He said: 'Will you accept Diyah?' He said: 'No.' He said: 'Will you kill him?' He said: 'Yes.' He said" 'Take him away (to kill him).' When he took him and turned away, he turned to those who were with him, and called him back, and said to him: 'Will you forgive him?' He said: No.' He said: 'Will you accept Diyah?' He said: No.' He said: 'Will you kill him?' He said: 'Yes.' He said: 'Take him away.' Then the Messenger of Allah said: 'If you forgive him, he will carry your sin and the sin of your companion (the victim).' So he forgave him and left him, and I was him dragging his string."