Al-Muhallab reported God’s Messenger as saying, “If the enemy attacks you at night let your war-cry be Ha’ Mim'. They will not be helped’.”*
*These words may be taken from sura 41 which is one beginning with Ha Mim. The other words are in verse 16 of that sura. Cf. 44: 41.
Tirmidhi and Abu Dawud transmitted it.