Musnad Ahmad 5:770

Aswad ibn Amir narrated to us, Abu Bakr informed us, from al-A’mash, from Salamah ibn Kuhayl, from Abdullah ibn Sabu’, who said: Ali, may Allah be pleased with him, delivered a sermon to us and said: "By the One who split the grain and created the soul, this [pointing to his beard] will surely be dyed by this [pointing to his head]." The people said: "Inform us who he is, for by Allah, we will surely exterminate his lineage." He said: "I adjure you by Allah that no one be killed except my killer." They said: "If you have known that, then appoint a successor." He said: "No, but I entrust you to that which the Messenger of Allah, may Allah bless him and grant him peace, entrusted you to."