Sahih Bukhari Volume 4, Book 52, Hadith Number 65.

Narated By Abu Musa : A man came to the Prophet 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 said, “He who fights that Allah’s Word (i.e. Islam) should be superior, fights in Allah’s Cause.”