Musnad Abu Ya'la al-Mawsili 115:108

Narrated to us Nasr, my father informed me, from al-Muthanna ibn Sa'id, from Qatadah, from Anas, who said: When the Prophet (peace and blessings of Allah be upon him) went on a military expedition, he would say: "O Allah, You are my strength and You are my helper, and by You I fight."