Musannaf Ibn Abi Shayba 37:328

Abu Usama narrated to us, from Awf, from al-Hasan, who said: A man came to al-Zubayr during the days of the Camel and said, "Shall I kill Ali for you?" He said, "And how?" He said, "I will go to him and inform him that I am with him, then I will assassinate him." Al-Zubayr said, "No, 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.'"