Someone who heard the Prophet ﷺ informed me that he said, "If you are attacked at night, let your battle cry be, 'Ha Mim, they shall not be granted victory.'"
حدثنا محمد بن كثير أخبرنا سفيان عن أبي إسحق
أخبرني من سمع النبي ﷺ يقول إن بيتم فليكن شعاركم حم لا ينصرون
[Sahih] Tirmidhi transmitted it in the Book of Jihad, chapter on what has been said about the war cry, hadith no. 1682, from the hadith of Sufyan Thawri through this chain. Hakim graded it sahih according to the criteria of Bukhari and Muslim, 2/107, and Dhahabi agreed with him. Abu Ishaq explicitly stated that he heard it directly, as found in Abdul-Razzaq, 5/233, hadith.