Sahih Muslim Book 20, Hadith Number 4687.

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

It has been narrated through a different chain of transmitters on the same authority, i.e. Abu Musa Ash’ari, that a man asked the Messenger of Allah (may peace be upon him) about fighting in the way of Allah, the Exalted and Majestic, a man who fights out of rage or out of family pride. He raised his head towards him-and he did so because the man was standing and said: Who fights that the word of Allah be exalted fights in the way of Allah.