Musnad Ahmad 769:13

Isma'il—who is Ibn Ibrahim—narrated to us, Abu Hayyan narrated to us, he said: I heard a sheikh in Medina narrating that 'Abdullah ibn Abi Awfa wrote to 'Ubaydullah when he intended to fight the Haruriyyah. I said to his scribe—and he was a friend of mine—"Copy it for me," and he did. [The letter stated]: "Indeed, the Messenger of Allah (peace and blessings of Allah be upon him) used to say: 'Do not wish to meet the enemy, and ask Allah, the Almighty and Majestic, for well-being (al-'afiyah). But if you meet them, then be patient, and know that Paradise is under the shadows of the swords.'" He said: Then he would look, and when the sun passed its zenith, he would advance toward his enemy. Then he would say: "O Allah, Revealer of the Book, Mover of the clouds, and Defeater of the confederates, defeat them and grant us victory over them."