The Book on Government (Kitab Al-Imara)
Muslim :: Book 20 : Hadith 4684
It has been narrated on the authority of Abu Musa Ash'ari that a desert Arab came to the Holy Prophet (may peace be upon him) and said: Messenger of Allah, one man fights fgr the spoils of war; another fights that he may be remembered, and another fights that he may see his (high) position (achieved as a result of his valour in fighting). Which of these is fighting in the cause of God? The Messenger of Allah (may peace be upon him) said: Who fights so that the word of Allah is exalted is fighting in the way of Allah.