Musannaf Ibn Abi Shayba 18:1273

Abu Bakr narrated to us, he said: Abu Usama narrated to us, from Awf, from Hamza ibn Umar, from Alqama ibn Wa’il, from his father, who said: I witnessed the Messenger of Allah (peace and blessings of Allah be upon him) when the killer was brought, being dragged by his leather strap. The Messenger of Allah (peace and blessings of Allah be upon him) said to the guardian of the slain person: "Will you pardon him?" He said: "No." He said: "Will you take the blood money?" He said: "No." He said: "Will you kill him?" He said: "Yes." He repeated it to him three times. Then the Messenger of Allah (peace and blessings of Allah be upon him) said to him: "If you pardon him, he will bear his sin." He said: "So he pardoned him." He said: "And I saw him being dragged by his leather strap, having been pardoned."