Musnad Ahmad 31:689

'Affan narrated to us, Shu'bah narrated to us, he said: Sa'd ibn Ibrahim informed us, he said: I heard Abu Umamah ibn Sahl ibn Hunayf narrating from Abu Sa'id, that when the people of Qurayzah submitted to the judgment of Sa'd ibn Mu'adh, the Messenger of Allah (peace and blessings of Allah be upon him) sent for him. He came on a donkey, and the Messenger of Allah (peace and blessings of Allah be upon him) said: "Stand up for your leader," or "for the best of you." He said: "Indeed, these people have submitted to your judgment." He said: "I judge that their fighters be killed and their offspring be taken captive." He said: "You have judged with the judgment of the King."