Musannaf Ibn Abi Shayba 18:1102

Abu Bakr narrated to us, he said: Abd al-Rahim narrated to us, from al-Hasan ibn Amr, from Fudayl, from Ibrahim, he said: Each man among them shall be made to swear by Allah, "I did not kill, nor do I know the killer," then he shall pay the blood money for him.