Sahih Bukhari Volume 001, Book 003, Hadith Number 125.
Narated By Abu Musa : A man came to the Prophet and asked, "O Allah's Apostle! What kind of fighting is in Allah's cause? (I ask this), for some of us fight because of being enraged and angry and some for the sake of his pride and haughtiness." The Prophet raised his head (as the questioner was standing) and said, "He who fights so that Allah's Word (Islam) should be superior, then he fights in Allah's cause."