Musnad Ahmad 32:966

'Abd al-Rahman ibn Mahdi narrated to us, al-Muthanna ibn Sa'id narrated to us, from Qatadah, from Anas, from the Prophet, peace and blessings of Allah be upon him, who said: "If one of you sleeps through the prayer, or is heedless of it, let him pray it when he remembers it, for Allah says: {Establish the prayer for My remembrance} [Ta-Ha: 14]." He said: And when the Prophet, peace and blessings of Allah be upon him, would go on a military expedition, he would say: "O Allah, You are my strength, and You are my helper, and by You I fight."