Musnad Ahmad 7:22

'Affan narrated to us, Mubarak narrated to us, al-Hasan narrated to us, he said: A man came to al-Zubayr ibn al-'Awwam and said: "Shall I not kill 'Ali for you?" He said: "No. And how would you kill him while he has soldiers with him?" He said: "I will join him and assassinate him." He said: "No. Indeed, the Messenger of Allah, peace and blessings of Allah be upon him, said: 'Indeed, faith is a restraint against assassination; a believer does not assassinate.'"