This letter was written by Khalid, from his head-quarters in Babylonia, to the Persian monarch Emperor Yazdegerd III before invading it. (History of the World, Volume IV [Book XII. The Mohammedan Ascendency], page 463, by John Clark Ridpath, LL.D. 1910.
Chálid ibn al-Valíd: Citáty anglicky
As quoted in [Philip K. Hitti, History of The Arabs, 130]