Sunan an-Nasa'i 4723

It was narrated from 'Alqamah binWa'il Al-Hadrami that his farther said:A man who had killed someone was brought to the Messenger of Allah, and he was brought by the heir of the victim. The Messenger of Allah said to him. 'Will you forgive him? He said: No.' He said: 'Will you kill him? He said: 'Yes.' He said: 'Go away.' Then when he went away, he called him back and said: will you forgive him?' He said: 'No.' He said: 'Will you accept the Diyah? He said: 'No.' He said: 'will you kill him? He said: 'Yes.' He said: 'Go away.' Then when he had gone he said: If you forgive him, he will carry your sin and the sin of your companion (the victim)." So he forgave him and let him go." He said: "And I saw him dragging his string