Musannaf Ibn Abi Shayba 18:878

Abu Bakr narrated to us, he said: Ibn ‘Uyaynah narrated to us, from Ibn Tawus, who said: I said to my father: "A man is killed, then he pardons his blood (the retaliation for his murder)." He said: "It is permissible." I said: "Whether it was by mistake or intentional?" He said: "Yes."