 Complete the following phrase:  Rostam, while preparing to face the Arab army, wrote a letter to his brother Farrukhzad, telling him to gather an army and then go to Azerbaijan where he should pray for him. Rostam also reminded Farrukhzad that Yazdegerd III was the only legacy left from the Sasanians. Rostam then set out from Ctesiphon in command of a large Sasanian force to confront the Arab-Muslim army of Caliph Umar on the western bank of the Euphrates River at the plains of 