Musnad Abu Ya'la al-Mawsili 3:78

Ubayd Allah narrated to us, Yazid bin Zuray' narrated to us, Sa'id bin Abi Arubah narrated to us, from Qatadah, from al-Hasan, from Qays bin 'Abbad, who said: I went to Ali, I and another man. I said to him: "Did the Messenger of Allah (peace be upon him) entrust anything to you that he did not entrust to anyone else?" He said: "No, except for what is in this sheath of mine." He then brought out a document. In that document of his, it was written: "The believers are equal in their blood; the lowest of them strives for their protection, and they are a united hand against those who are outside of them. No believer shall be killed for a disbeliever, nor one who has a covenant 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."