The Muslim armies reached there in July 636. A week or two later, around mid July, the Byzantine army arrived. The Byzantine commander in chief, Vahan, sent Christian Arab troops of the Ghassanid king, Jabalah ibn al-Aiham, to check the strength of the Muslims. Khalid's mobile guard defeated and routed the Christian Arabs; this was the last action before the battle started. For the next month negotiations continued between the two armies, and Khalid went to meet Vahan in person at Byzantine camp. Meanwhile, the Muslims received reinforcements sent by Caliph Umar.