Alqamah bin Wa'il bin Hujr al-Hadrami (may Allah have mercy on him) narrates from his father (Sayyiduna Wa'il bin Hujr, may Allah be pleased with him) that a killer was brought to the Messenger of Allah (peace and blessings be upon him). He (peace and blessings be upon him) asked the heir of the slain: "Do you pardon him?" He said: No. Then he asked: "Are you content with blood money?" He said: No. He asked: "Will you kill him?" He said: Yes. He said: "Very well then, take him away." He took him away; then he (peace and blessings be upon him) called him back and said: "If you pardon him, your sins and the sins of the slain will be upon him." So he pardoned him. He said: I saw him going off dragging his strap.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 16075
Amr bin Shu'ayb narrates from his father, and he from his grandfather (may Allah be pleased with him), that the Prophet (peace and blessings be upon him) said: "Whoever kills deliberately is to be handed over to the heir of the slain; if he wishes he may take blood money from him, and if he wishes he may kill him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 16076