Musnad Ahmad 5:429

Yahya narrated to us, Sa'id ibn Abi 'Arubah narrated to us, from Qatadah, from al-Hasan, from Qays ibn 'Abbad, who said: I and al-Ashtar went to 'Ali and said: "Did the Prophet of Allah, may Allah bless him and grant him peace, entrust you with anything that he did not entrust to the people in general?" He said: "No, except for what is in this book of mine." He said: And it was a book in the scabbard of his sword. In it was: "The blood of the believers is equal, and they are a single hand against those who are other than them, and the lowest of them strives for their protection. Beware, no believer shall be killed for a disbeliever, nor shall one who has a covenant be killed while in his covenant. Whoever commits an innovation or harbors an innovator, upon him is the curse of Allah, the angels, and all of mankind."