Musnad Ahmad 658:103

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Abu Ishaq, who said: I heard Al-Bara' say: The Messenger of Allah, peace and blessings of Allah be upon him, on the day of the Confederates (Al-Ahzab), was carrying the dirt with us. The dirt had covered the whiteness of his stomach, while he was saying: "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, for the people have transgressed against us." And sometimes he would say: "For the assembly has refused us; if they intend a trial, we refuse." And he would raise his voice with it.