Sahih Bukhari
One-fifth of Booty to the Cause of Allah (Khumus)
Volume: 004 / Book Number: 053 / Hadith Number: 355
Narrated 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."