Muhallab bin Abi Sufrah (may Allah have mercy on him) said: One who heard the Messenger of Allah (peace and blessings be upon him) informed me that he would say: "If you spend the night (facing the enemy), let your battle-cry be 'Ha Mim, la yunsarun' (Ha Mim, they will not be helped)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الفيء والغنيمة / 13054