Mishkat al-Masabih 19 Jihad كتاب الجهاد 160

وَعَن الْمُهلب أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: ” إِنْ بَيَّتَكُمُ الْعَدُوُّ فَلْيَكُنْ شِعَارُكُمْ: حم لَا ينْصرُونَ ” . رَوَاهُ التِّرْمِذِيّ وَأَبُو دَاوُد

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.