Hadith #4499 (Sunan Abi Dawud)
Book: Types of Blood-Wit (Kitab Al-Diyat)
Narrator(s): Alqama bin Wa'il bin Hjr علقمة بن وائل بن حجر, Muhammad bin Awf bin Sufyan محمد بن عوف بن سفيان الطائي, Smak bin Hrb bin Aws سماك بن حرب بن أوس, Yazid bin Ata'a bin Yazid يزيد بن عطاء بن يزيد
Grade: Sahih

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.

Source: https://parallelquran.com/hadith/sunan-abi-dawud-4499/