Mark Whittow: The Making of Byzantium: 600–1025. (PDF; 26 MB) University of California Press, Berkley 1996, S. 195, 203, 215: “… Azerbaijan was the scene of frequent anti-caliphal and anti-Arab revolts during the eighth and ninth centuries, and Byzantine sources talk of Persian warriors seeking refuge in the 830s from the caliph’s armies by taking service under the Byzantine emperor Theophilos. […] Azerbaijan had a Persian population and was a traditional centre of the Zoroastrian religion. […] The Khurramites were a […] Persian sect, influenced by Shiite doctrines, but with their roots in a pre-Islamic Persian religious movement …”
