A man came to Zubayr bin Awwam and said, "Shall I kill Ali for you?"Zubayr replied, "No. How would you kill him when he has armies with him?"The man said, "I will catch up with him and assassinate him by stealth."Zubayr said, "No. The Messenger of Allah ﷺ said, 'Faith restrains treacherous killing. A believer does not assassinate by stealth.'"
حدثنا عفان حدثنا مبارك
جاء رجل إلى الزبير بن العوام فقال أقتل لك عليا قال لا وكيف تقتله ومعه الجنود قال ألحق به فأفتك به قال لا إن رسول الله ﷺ قال إن الإيمان قيد الفتك لا يفتك مؤمن
Its isnad is sahih. Mubarak bin Fadala is trustworthy. Ibn Main deemed him trustworthy on one occasion and daif on another, while Hushaym deemed him trustworthy, and Affan used to praise him highly and affirm his trustworthiness. Abu Zura said, "He engages in tadlis frequently, but when he says 'he narrated to us,' he is trustworthy." This is the fair assessment of him.The hadith appears in Majma al-Zawaid 1:96, where it is stated, "Ahmad narrated it, and among its narrators is Mubarak bin Fadala, who is trustworthy but engages in tadlis. However, he said, 'Hasan narrated to us.'" This hadith will come again right after this one as number 1427, and it will come a third time as number 1433, narrated by Ayub from Hasan, so Mubarak was not the sole narrator of it.See also Tarikh Bukhari al-Kabir 1/2/288, where it states, "Khalid bin Yusuf bin Khalid narrated to me from Yazid bin Zuray from Hasan, 'I was informed that a man...'"