This article introduces into scientific circulation the material of a unique manuscript of the work “Risale-yi Babiye” (“Derbent Epistle”) – a little-studied source on the history of Islam in the North-Eastern Caucasus. The Turkish poet ‘Adli (d. 1618) began work on this writing, which is an abridged translation of a much older Persian source (protograph), in Bab al-Abwab (Derbent) in 1581. The text of the source contains historical information and hadith material that extol military service in Bab al-Abwab and the protection of the northern borders of the Caliphate from attacks from the North.“Risale-yi Babiye” by ‘Adli contains previously unknown historical information relating to the period of early Islam in the North-Eastern Caucasus (7th-10th centuries). We are talking about a more accurate understanding of the Arabic designation of “the borderland of Bab al-Abwab” (as-sagr bi-Bab al-Abwab), the status and specific personalities of the amirs of the border fortresses. Information about the gate “Bab az-zam” in the northern wall of Bab al-Abwab, as well as their location, deserves special attention. No less interesting is the designation in the Turkish-Ottoman work of “the wicked and fearless Turks” as enemies of Muslims. It is noteworthy that the local Persian-language prototype of the work, created during the Arab-Khazar wars, does not call them Khazars, like almost all Arabic sources, but, following its original source, designates them as “Turks from Turkestan”. This fact once again confirms our earlier hypothesis that the Turkic Khaganate did not disappear or disintegrate in the 7th century, allegedly under the pressure of the Hunnic Khazar clan. After the military alliance between the Turkuts and the Hunno-Savirs (“Khazars”) of the Caspian region concluded in 657, the Arabs began to call the state created by the Turkuts from the Ashina clan the Khazar Khaganate.