Sahih Bukhari Volume 004, Book 053, Hadith Number 355.

Narated By Abu Musa Al-Ashari : A bedouin asked the Prophet, "A man may fight for the sake of booty, and another may fight so that he may be mentioned by the people, and a third may fight to show his position (i.e. bravery); which of these regarded as fighting in Allah's Cause?" The Prophet said, "He who fights so that Allah's Word (i.e. Islam) should be superior, fights for Allah's Cause."