Musnad Abu Ya'la al-Mawsili 115:292

Ubayd Allah narrated to us, Abd al-Rahman ibn Mahdi narrated to us, al-Muthanna ibn Sa'id narrated to us, from Qatadah, from Anas, who said: When the Messenger of Allah (peace be upon him) would meet the enemy, he would say: "O Allah, You are my strength and my helper, and by You I fight."