Musannaf Ibn Abi Shaybah 29917 — One who kills a man by mistake

Ibn Jurayj says that Ata said: "If a man kills a man by mistake and then kills another man deliberately, he is to pay the blood money for the mistaken killing, because its blood money had been established before the deliberate killing." A person asked him: "And if someone kills deliberately and then kills by mistake?" He said: "He does not pay the blood money, because his blood has indeed become forfeit."