Sahih Muslim Book 20, Hadith Number 4686.

Chapter : One who fights that the word of Allah is exalted fights in the way of Allah.

It has been narrated (through yet another chain of transmitters) on the same authority, i.e. Abu Musa, who said: We, came to the Messenger of Allah (may peace be upon him) and said: Messenger of Allah, one of us who fights to display his valour… (followed by the same words as we have in the previous tradition).