Musannaf Ibn Abi Shayba 28:1045

Sulayman ibn Harb narrated to us, he said: Ghalib ibn Sulayman—or Salih—narrated to us, he said: al-Zubayr ibn [al-Sarraf] narrated to us, he said: Mus'ab ibn al-Zubayr said to us while we were facing al-Mukhtar: "Let your watchword be 'Ha-Mim, La Yunsarun' (Ha-Mim, they shall not be helped), for it was the watchword of the Prophet, may Allah bless him and grant him peace."