Qays bin Abbad (may Allah have mercy on him) said: We came to Sayyiduna Ali (may Allah be pleased with him), and Jariyah bin Qudamah al-Sa'di (may Allah be pleased with him) was with me. We asked: Do you have any covenant from the Messenger of Allah (peace and blessings be upon him)? He said: No, only what is in the sheath of my sword. Then he took out a letter from it and read it, and in it was written: "The blood of the Muslims is equal, one to another; the protection granted by the lowest of them is honoured; and they will be helped against those besides them. Beware! A Muslim is not to be killed in return for a disbeliever, nor one who has a covenant while his covenant lasts. Whoever introduces an innovation or gives shelter to an innovator, upon him is the curse of Allah, the angels and all people."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15910