Musnad Ahmad 27:932

Hassan narrated to us, Zuhair narrated to us, from Bayan, from Wabarah, from Sa’id ibn Jubayr, who said: Abdullah ibn Umar came out to us while we were hoping that he would narrate to us a hadith or a good hadith. A man among us, who was called al-Hakam, hastened to us and said: "O Abu Abd al-Rahman, what do you say about fighting during the *fitnah* (civil strife)?" He said: "May your mother be bereaved of you! Do you know what the *fitnah* is? Muhammad, peace and blessings of Allah be upon him, used to fight the polytheists, and entering into them or into their religion was a *fitnah*. It is not like your fighting over kingship."