Musnad Ahmad 658:82

Ibn Numayr narrated to us, Ajlah narrated to us, from Abu Ishaq, from al-Bara’ ibn ‘Azib, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said to us: "You will surely meet the enemy tomorrow, and your watchword shall be: 'Ha-Mim, they will not be helped.'"