In the Muslim world, several expeditions were undertaken to try to find and study Alexanders's wall, specifically the Caspian Gates of Derbent. An early expedition to Derbent was ordered by the Caliph Umar (586–644 AD) himself, during the Arab conquest of Armenia where they heard about Alexander's Wall in Derbent from the conquered Christian Armenians. Umar's expedition was recorded by the renowned exegetes of the Quran, Al-Tabarani (873–970 AD) and Ibn Kathir (1301–1373 AD), and by the Muslim geographer Yaqut al-Hamawi (1179–1229 AD):