Musannaf Ibn Abi Shayba 36:294

Yazid ibn Harun narrated to us, Hisham informed us, from Muhammad, who said: Huyayy ibn Akhtab made a covenant with the Messenger of Allah (may Allah bless him and grant him peace) that he would not support anyone against him, and he made Allah a guarantor over him. He said: When the day of Qurayza came, he was brought, along with his son, Salma. He said: The Messenger of Allah (may Allah bless him and grant him peace) said, "Fulfill the measure." So he ordered him, and his neck was struck, as was the neck of his son.