Musannaf Ibn Abi Shayba 36:265

Yahya ibn Adam narrated to us, he said: Zuhayr narrated to us, from Abu Ishaq, he said: I heard al-Muhallab ibn Abi Sufra saying, and he mentioned the Haruriyyah and their night attacks. He said: The companions of Muhammad said: The Messenger of Allah (may Allah bless him and grant him peace) said on the day of the Trench, while he feared that Abu Sufyan would launch a night attack against them: "If you are attacked at night, then your password shall be 'Ha-Mim', they will not be aided."