Musnad Ahmad 1426, 1427
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.”

Al-Hasan said:  A man came to az-Zubair bin al-`Awwam (رضي الله عنه) and said: “Shall I not kill `Ali for you?” He said: “How can you kill him when he has people with him?” (and he narrated a similar report.)
حَدَّثَنَا عَفَّانُ، حَدَّثَنَا مُبَارَكٌ، حَدَّثَنَا الْحَسَنُ، قَالَ جَاءَ رَجُلٌ إِلَى الزُّبَيْرِ بْنِ الْعَوَّامِ فَقَالَ أَقْتُلُ لَكَ عَلِيًّا قَالَ لَا وَكَيْفَ تَقْتُلُهُ وَمَعَهُ الْجُنُودُ قَالَ أَلْحَقُ بِهِ فَأَفْتِكُ بِهِ قَالَ لَا إِنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ إِنَّ الْإِيمَانَ قَيْدُ الْفَتْكِ لَا يَفْتِكُ مُؤْمِنٌ حَدَّثَنَا يَزِيدُ بْنُ هَارُونَ أَنْبَأَنَا مُبَارَكُ بْنُ فَضَالَةَ حَدَّثَنَا الْحَسَنُ قَالَ أَتَى رَجُلٌ الزُّبَيْرَ بْنَ الْعَوَّامِ فَقَالَ أَلَا أَقْتُلُ لَكَ عَلِيًّا قَالَ وَكَيْفَ تَسْتَطِيعُ قَتْلَهُ وَمَعَهُ النَّاسُ فَذَكَرَ مَعْنَاهُ‏.‏