Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) narrates that the people of Banu Qurayzah accepted Sayyiduna Sa'd (may Allah be pleased with him) as arbiter. The Messenger of Allah (peace and blessings be upon him) sent for him, and when he came, he (peace and blessings be upon him) said: "Stand up for your leader, or the best of you." Then Sayyiduna Sa'd (may Allah be pleased with him) sat beside the Messenger of Allah (peace and blessings be upon him), and he (peace and blessings be upon him) said: "Sa'd! These people have agreed to your judgement." He said: My judgement is that their fighters be killed.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18184