Sahih Muslim Book 19, Hadith Number 4368.

Chapter : Justification for killing those guilty of breach of trust and making the people of the fort surrender on the arbitration of a just person.

It has been narrated on the authority of Abu Sa’id al-Khudri who said: The people of Quraiza surrendered accepting the decision of Sa’d b. Mu’adh about them. Accordingly, the Messenger of Allah (may peace be upon him) sent for Sa’d who came to him riding a donkey. When he approached the mosque, the Messenger of Allah (may peace be upon him) said to the Ansar: Stand up to receive your chieftain. Then he said (to Sa’d): These people have surrendered accepting your decision. He (Sa’d) said: You will kill their fighters and capture their women and children. (Hearing this), the Prophet (may peace he upon him) said: You have adjudged by the command of God. The narrator is reported to have said: Perhaps he said: You have adjudged by the decision of a king.

Ibn Muthanna (in his version of the tradition) has not mentioned the alternative words.