Musannaf Ibn Abi Shayba 28:1041

Waki’ narrated to us, he said: ‘Ikrimah ibn ‘Ammar narrated to us, from Iyas ibn Salamah, from his father, who said: We went on a military expedition with Abu Bakr against Hawazin, and our watchword was: "Ammit, Ammit" (Kill, kill).