History of the Prophets and Kings
The Khazars and the Arabs came into conflict as a result of the first phase of Muslim expansion: by 640, the Arabs had reached Armenia, and in 642, they launched their first raid across the Caucasus under Abd ar-Rahman ibn Rabiah. This raid reached Derbent, where, according to the "History of the Prophets and Kings" of al-Tabari, the Persian governor, Shahrbaraz, offered to surrender the fortress to the Arabs and even to aid them against the unruly native Caucasian peoples, if he and his followers were relieved of the obligation to pay the "jizya" tax. The proposal was accepted and ratified by Caliph Umar ().