Musnad Ahmad 31:180

Muhammad narrated to us, Shu'bah narrated to us, from Sa'd ibn Ibrahim, from Abu Umamah ibn Sahl, who said: I heard Abu Sa'id al-Khudri say: The people of Qurayza submitted to the judgment of Sa'd ibn Mu'adh. He said: The Messenger of Allah, peace and blessings of Allah be upon him, sent for Sa'd, and he came to him on a donkey. He said: When he drew near to the mosque, the Messenger of Allah, peace and blessings of Allah be upon him, said: "Stand up for your leader" — or "your best one." Then he said: "Indeed, these people have submitted to your judgment." He said: "Kill their fighters and take their offspring captive." He said: The Prophet, peace and blessings of Allah be upon him, said: "You have judged with the judgment of Allah." And perhaps he said: "You have judged with the judgment of the King."