Musnad Ahmad 658:212

Husayn ibn Muhammad narrated to us, Isra'il narrated to us, from Abu Ishaq, from al-Bara' ibn 'Azib, who said: I saw the Prophet, may Allah bless him and grant him peace, carrying the dirt of the Trench until the dirt covered the skin of his abdomen, while he was reciting the rajaz verses of 'Abdullah ibn Rawahah: "O Allah, were it not for You, we would not have been guided, nor would we have given charity, nor would we have prayed. So send down tranquility upon us, and make firm the feet if we encounter [the enemy]. Indeed, those who have transgressed against us, and if they intend sedition, we refuse."