Musnad Ahmad 27:1240

Hisham ibn Sa'id narrated to us, Khalid—meaning al-Tahhan—narrated to us, Bayan narrated to us, from Wabarah, from Ibn Jubayr—meaning Sa'id—who said: Ibn Umar came out to us while we were hoping that he would narrate to us a hadith that would please us. A man hurried to him and said: "O Abu Abd al-Rahman, what do you say about fighting during the *fitnah* (civil strife)? For Allah, the Almighty and Majestic, said: 'And fight them until there is no *fitnah*' [Al-Baqarah: 193]." He said: "Woe to you! Do you know what the *fitnah* is? The Messenger of Allah (peace and blessings of Allah be upon him) used to fight the polytheists, and entering into their religion was a *fitnah*. It is not like your fighting over kingship."