From one who heard the Prophet ﷺ saying: "If you suffer a surprise attack from the enemy then say: 'Ha Min, they will not be victorious.'"
سَمِعَ النَّبِيَّ ﷺ يَقُولُ إِنْ بَيَّتَكُمُ الْعَدُوُّ فَقُولُوا حم لاَ يُنْصَرُونَ
Al-Muhallab ibn AbuSufrah said: A man who heard the Prophet ﷺ say: If the enemy attacks you at night, let your war cry be Ha-Mim. They will not be helped.
سَمِعَ النَّبِيَّ ﷺ يَقُولُ إِنْ بُيِّتُّمْ فَلْيَكُنْ شِعَارُكُمْ حم لاَ يُنْصَرُونَ