Musnad Ahmad 7:29

Isma'il narrated to us, Ayyub narrated to us, from al-Hasan, who said: A man said to al-Zubayr: "Shall I not kill 'Ali for you?" He said: "How would you kill him?" He said: "I will assassinate him (fatk)." He said: "No. The Messenger of Allah, peace and blessings of Allah be upon him, said: 'Faith is a restraint against assassination (fatk); a believer does not assassinate.'"