Musannaf Ibn Abi Shayba 38:57

Abu Usama narrated to us, from Awf, from al-Hasan, who said: A man came to al-Zubayr on the day of the Camel and said: "Shall I kill Ali for you?" He said: "And how?" He said: "I will come to him and inform him that I am with him, then I will assassinate him." Al-Zubayr said: "I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'Faith is a restraint against assassination; a believer does not assassinate.'"