Islamic influence on China

I am puzzling over the Islamic influence on Central Asia, prompted by a trip to Kashgar where there are a number of obvious influences, particularly mosques. A useful (and authoritative?, given the author's pedigree) travel log explains the origins: "In the year of 622 of the Common Era (C.E.), Muhammad made his famous Hejira (emigration) to Medina from his birthplace Mecca, where he had achieved only very moderate success in winning converts to the new faith as revealed to him by Allah (God). When he died 10 years later, the strictly monotheistic religion he had founded had not only been accepted by most tribes in the Arabian Peninsula, but had also forged a new Arab state with Medina as the source of its spiritual and political power. Motivated by the new faith and strengthened by the new state organization, the Arab people set out on a path of conquest and established, within 100 years of MuhammadÂ?s death, a vast empire spreading out from Spain in the west to the borders of China in the east. "