Musnad Ahmad 5:427

'Abdullah narrated to us, 'Ubaydullah ibn 'Umar al-Qawariri narrated to me, Muhammad ibn 'Abd al-Wahid ibn Abi Hazm narrated to us, 'Umar ibn 'Amir narrated to us, from Qatadah, from Abu Hassan, from 'Ali, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "The blood of the believers is equal in value, and they are a single hand against those who are other than them. The lowest of them strives to fulfill their protection (dhimmah). Beware, a believer shall not be killed for a disbeliever, nor shall one who has a covenant be killed while in his covenant."