Muhammad bin Ali (may Allah have mercy on him) said that a scroll was found in the sheath of the Messenger of Allah (peace and blessings be upon him), on which was written: "May Allah curse the one who kills someone other than the killer, and strikes someone other than the one who struck him; and whoever turns away from his master has disbelieved in what was revealed to Muhammad (peace and blessings be upon him)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15895