Musnad Ahmad 28:240

Abu al-Nadr and Abd al-Samad narrated to us, they said: Muhammad—meaning Ibn Rashid—narrated to us, Sulayman narrated to us, from ‘Amr ibn Shu‘ayb, from his father, from his grandfather: "That the Messenger of Allah (peace and blessings of Allah be upon him) decreed that the blood money (diya) of the People of the Two Books is half the blood money of the Muslims, and they are the Jews and the Christians."