Sayyiduna al-Bara' (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) was carrying earth with us on the day of al-Ahzab (the Confederates). Then he narrated the (preceding) hadith.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13294