Musannaf Ibn Abi Shayba 36:300

Kathir ibn Hisham narrated to us, from Ja'far, who said: Yazid ibn al-Asamm narrated to us, who said: When Allah uncovered the Confederates (al-Ahzab) and the Prophet, peace and blessings of Allah be upon him, returned to his house and began to wash his head, Gabriel came to him and said: "May Allah pardon you, you have laid down your weapons, while the angels of the heaven have not laid down theirs. We have come to the fortress of Banu Qurayza." Then the Messenger of Allah, peace and blessings of Allah be upon him, called out to the people to come to the fortress of Banu Qurayza. Then the Messenger of Allah, peace and blessings of Allah be upon him, performed ghusl (ritual bath), and then he came to them at the fortress.