Musannaf Ibn Abi Shayba 3:5884

Mu'tamir narrated to us, from Layth, from Mujahid and al-Hakam, who both said: When it is at the time of the skirmish and the drawing of swords, it suffices for a man that his prayer be [the saying of] "Allahu Akbar." If there is nothing but one "Allahu Akbar," it suffices him, wherever his face may be turned.