Alqamah bin Wa'il narrates from his father (Wa'il bin Hujr, may Allah be pleased with him): I was with the Messenger of Allah (peace and blessings be upon him) when a killer was brought, from whom retaliation was being sought. The Messenger of Allah (peace and blessings be upon him) asked the heirs of the slain: "Do you pardon?" They said: "No." He (peace and blessings be upon him) asked: "Do you want to take blood money?" They said: "No." He (peace and blessings be upon him) asked: "Do you want to kill him?" They said: "Yes." So he (peace and blessings be upon him) said: "Then take him away." When they had gone, he (peace and blessings be upon him) called them back and again asked about pardon and blood money, and they gave the same answer. This happened three times; then the fourth time he (peace and blessings be upon him) said: "If you pardon him, he will bear your sin and the sin of your companion." So he let him go. Wa'il bin Hujr (may Allah be pleased with him) said: I saw him dragging his strap. And Yahya al-Qattan (may Allah have mercy on him) said: "He will bear his sin and the sin of your companion."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 16054