Musnad Ahmad 371:1

Aswad ibn Amir narrated to us, he said: Sharik narrated to us, from Abu Ishaq, from al-Muhallab ibn Abi Sufrah, from a man among the companions of the Prophet, may Allah bless him and grant him peace, from the Prophet, may Allah bless him and grant him peace, who said: "I do not see them tonight except that they will launch a night attack upon you. If they do so, then your watchword shall be: 'Ha-Mim, they will not be aided.'"