Sahih BukhariVolume 6, Book 60, Number 174

Narrated Said 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."