Musannaf Ibn Abi Shayba 28:1047

Narrated to us Abd al-Rahim ibn Sulayman, from al-Ajlah, from Abu Ishaq, from al-Bara’ who said: The Messenger of Allah (ﷺ) said: "You will meet the enemy tomorrow, so let your watchword be 'Ha-Mim'. They will not be aided."