Musannaf Ibn Abi Shayba 18:281

Abu Bakr narrated to us, he said: Abd al-Rahim narrated to us, from Hajjaj, from Makhul, from Zayd, that he said regarding the fingers: In every joint is one-third of the blood money of the finger, except for the thumb, for in it is half of its blood money if it is cut from the joint, because it has two joints.