Musannaf Ibn Abi Shayba 28:896

Waki’ narrated to us, he said: ‘Imran bin Hudair narrated to us, from Abu Mijlaz, that the Prophet (peace and blessings of Allah be upon him), when he met the enemy, would say: "O Allah, You are my strength and my helper. By You I move, by You I attack, and by You I fight."