Musnad Abu Ya'la al-Mawsili 115:63

Musa ibn Muhammad ibn Hayyan al-Basri narrated to us, Abd al-Rahman narrated to us, from al-Muthanna ibn Sa'id, from Qatadah, from Anas: That the Messenger of Allah, may Allah bless him and grant him peace, used to say when he went on a military expedition: "O Allah, You are my strength and You are my helper, and by You I fight."