Musnad Ahmad 1426

Al-Hasan narrated: A man came to az-Zubair bin al-’Awwam (رضي الله عنه) and said: “Shall I kill "Ali for you?” He said: “No; how can you kill him when he has the troops with him?” He said: “I will catch up with him and assassinate him.” He said: “No. The Messenger of Allah (ﷺ) said: Faith prohibits assassination; no believer should resort to that.”