Sayyiduna Ibn Umar (may Allah be pleased with them both) was asked: "Where did the Messenger of Allah (peace and blessings be upon him) pray on the day of al-Ahzab?" He said: "He (peace and blessings be upon him) prayed inside the valley, near a ruin there. And the Messenger of Allah (peace and blessings be upon him) had given the people permission to go back, then he ordered me to call them, so I called them."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13313