Sayyiduna Abdullah bin Amr (may Allah be pleased with them both) said that on the day of the Trench, when people were digging the trench around Madinah, the Messenger of Allah (peace and blessings be upon him) took hold of a pickaxe, struck a blow with it and said: "With this blow Allah will open the treasures of Rome." Then he struck a second blow and said: "With this Allah will open the treasures of Persia." Then he struck a third blow and said: "With this blow Allah will bring the people of Yemen as helpers and supporters."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13683