Sa'id bin Jubayr (may Allah have mercy on him) said: Ibn Umar (may Allah be pleased with them both) came to us and we hoped that he would narrate an excellent hadith to us. A man named Hakim said: O Abu Abd al-Rahman, what do you say about fighting in a fitnah? He said: May your mother be bereaved of you! Do you even know what fitnah is? Muhammad (peace and blessings be upon him) used to fight the polytheists, and entering among them (or: into their religion) was the fitnah; it was not like your fighting over kingship.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16807