Musannaf Ibn Abi Shayba 18:1049

Abu Bakr narrated to us, he said: Muhammad ibn Yazid narrated to us, from Abu al-Ala, from Abu Hashim, from Ibrahim, from Qatadah, from al-Hasan, who said regarding a man who killed intentionally, and the killer sat to be retaliated against for the killed person, then a man came and killed the killer by mistake: He said: His blood money is for the family of the one who was detained [for retaliation]. And Ata said: [It is] for the family of the first killed person.