Musannaf Ibn Abi Shayba 18:918

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: ‘Imran bin Hudayr narrated to us, from Abu Mijlaz, who said: If a man falls upon another man and one of them dies, the remaining one is liable. If you ask why, he said: Because the blood of a Muslim is not rendered void.