Narrated Sa`id bin Jubair:
Ibn `Umar came to us and a man said (to him), "What do you think about 'Qit-alal-Fitnah' (fighting caused by afflictions)." Ibn `Umar said (to him), "And do you understand what an affliction is? Muhammad used to fight against the pagans, and his fighting with them was an affliction, (and his fighting was) not like your fighting which is carried on for the sake of ruling."
USC-MSA web (English) reference: Vol. 6, Book 60, Hadith 174
Arabic reference: Book 65, Hadith 4651