Musannaf Ibn Abi Shayba 37:147

Mu'awiyah ibn Hisham narrated to us, from Sufyan, from Salamah ibn Kuhayl, from Kathir ibn Numayr: He said: A man came with some men to Ali and said, "I saw these people threatening you, so they fled, and I caught this one." He (Ali) said, "Shall I kill someone who did not kill me?" He said, "He insulted you." He said, "Insult him or leave him."