It is narrated from Sayyiduna Bara' bin Azib (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Tomorrow you will meet the enemy, so let your battle-cry be 'Ha Mim, la yunsarun' (Ha Mim, they will not be helped)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الفيء والغنيمة / 13055