ابن حوقل
This has also been reported by reputed medieval historians such as Al-Tabari, Ibn Hawqal, Istakhri, Moqaddasi, Yaghubi, Masudi, and Mostowfi Qazvini as well. Al-Khwarizmi also mentions this in Chapter 6, Vol. 6, of his book "Mafatih-ol-Olum".