Musannaf Ibn Abi Shaybah

Hazrat Ash'ath narrates that two men together inflicted a wound on a man's head; one of them wounded up to the membrane of the brain, and the other wounded up to the bone of the head. It was not known, nor did either of them know, who had wounded up to the membrane of the brain and who had wounded up to the bone of the head. Regarding this, Hasan Basri stated: Each of them will be liable for half the blood money for the wound to the membrane of the brain and half the blood money for the wound to the bone of the head.