Musnad Ahmad 788:9

Yahya ibn Adam narrated to us, Zuhayr narrated to us, Mansur narrated to us, from Shaqiq, from Abu Musa, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever fights so that the word of Allah may be supreme, he is in the way of Allah, the Almighty and Majestic."