Musnad Ahmad 788:253

Ziyad ibn Abdullah—meaning al-Bakkai—narrated to us, he said: Mansur narrated to us, from Shaqiq ibn Salamah, from Abu Musa al-Ash'ari, who said: A man asked the Prophet, may Allah bless him and grant him peace, while he was bowing his head, and said: "O Messenger of Allah, what is fighting in the way of Allah the Exalted? For one of us fights out of tribal zeal, and fights out of anger; does he have a reward?" He said: So the Messenger of Allah, may Allah bless him and grant him peace, raised his head toward him—and had he not been standing, he would not have raised his head toward him—then he said: "Whoever fights so that the word of Allah may be the highest, he is in the way of Allah the Almighty and Majestic."