A man came to the Prophet صلى الله عليه وسلم and asked, “A man fights for war booty; another fights for fame and a third fights for showing off; which of them fights in Allah’s Cause?” The Prophet صلى الله عليه وآله وسلم said, “He who fights that Allah’s Word (i.e. Islam) should be superior, fights in Allah’s Cause.”

Bukhari. Narrated by Abu Musa.