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