Musnad Ahmad 1433
It was narrated that al-Hasan said:
A man said to az-Zubair: “Shall I kill `Ali for you?” He said: “How would you kill him?” He said: “I will assassinate him.”He said: “No, the Messenger of Allah (ﷺ) said: Faith prohibits assassination; no believer should resort to that.`
حَدَّثَنَا إِسْمَاعِيلُ، حَدَّثَنَا أَيُّوبُ، عَنِ الْحَسَنِ، قَالَ قَالَ رَجُلٌ لِلزُّبَيْرِ أَلَا أَقْتُلُ لَكَ عَلِيًّا قَالَ كَيْفَ تَقْتُلُهُ قَالَ أَفْتِكُ بِهِ قَالَ لَا قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ الْإِيمَانُ قَيْدُ الْفَتْكِ لَا يَفْتِكُ مُؤْمِنٌ‏.‏