Qays bin Ubad (may Allah have mercy on him) said: Al-Ashtar and I came to Sayyiduna Ali (may Allah be pleased with him) on the day of al-Jamal. I said: Did the Prophet (peace and blessings be upon him) entrust you with any covenant other than the general one? He said: "No, only this," and he took out (a document) from the sheath of his sword, in which was written: "The blood of the believers is equal; the protection granted by the lowest of them is accepted, and they are one hand against those besides them. A believer is not to be killed in retaliation for a disbeliever, nor one under a covenant (dhimmi) while his covenant is in force."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16813