Abdullah bin Umar came out to us while we were hoping he would relate to us a hadith, or perhaps a fine hadith. A man among us called Hakam beat us to it and asked, "Abu Abdul-Rahman, what do you say about fighting during civil strife?"He replied, "May your mother be bereaved of you. Do you even know what fitnah is? Muhammad ﷺ used to fight the polytheists, and entering among them or into their religion was the fitnah. It is nothing like your fighting for the sake of kingship."
حدثنا حسن حدثنا زهير عن بيان عن وبرة
خرج علينا عبد الله بن عمر ونحن نرجو أن يحدثنا حديثا أو حديثا حسنا فبدرنا رجل منا يقال له الحكم فقال يا أبا عبد الرحمن ما تقول في القتال في الفتنة قال ثكلتك أمك وهل تدري ما الفتنة إن محمدا ﷺ كان يقاتل المشركين فكان الدخول فيهم أو في دينهم فتنة وليس كقتالكم على الملك
Its chain of transmission is sahih. Bayan is Ibn Bishr Al-Ahmasi. Wabarah is Ibn Abdul-Rahman Al-Musali. Bukhari narrated the hadith in 233 through Zuhayr, and in 13:39 through Khalid bin Abdullah, both of them from Wabarah with similar wording, without naming the man who asked Ibn Umar. In Fath al-Bari it is mentioned that in the narration of Bayhaqi and the Mustakhraj of Abu Naeem, his name is given as Hakeem. It seems that the Hafiz had not seen the narration in the Musnad, or had forgotten it when he wrote that.