Musannaf Ibn Abi Shaybah

Hazrat Ibn Jurayj states that Hazrat Ata said: If a man kills someone by mistake and then intentionally kills another person, he will pay the blood money for the mistake because the blood money had already become obligatory due to the intentional killing. A person asked him: And if someone intentionally kills and then kills by mistake? He replied: He will not pay the blood money because, indeed, the compensation for his blood has not been given.