Ustadh Sis (also spelled: Ustad Sis, or Ostad Sis, Persian: استاذ سیس) was a Persian heresiarch and anti-Arab rebel leader. They write that he was once a governor of Khorasan and father to Ma'mūn’s Iranian mother, Marjil, which makes him Ma'mūn's grandfather.
He claimed he was a prophet of God in the eastern fringe of Khorasan in the mid 8th century and managed to gain followers among the villagers in that area. Many were previously followers of Bihafarid, whom the Abbasid commander, Abu Muslim, crushed militarily.
Reinvigoration of Bihafarid's movement
Ustadh Sis launched a rebellion in 767, purportedly with 300,000 fighting men. His initial base was the mountainous region of Badghis, and he soon occupied Herat and Sistan before marching towards Merv. He initially defeated an Abbasid army under the command of al-Ajtham of Merv, but was himself defeated in a bloody battle against an army led by al-Mahdi, the son of the Caliph al-Mansur (and a future Caliph himself).
Suppression of Ustadh Sis rebellion
According to al-Tabari, 70,000 of Ustadh Sis's followers were killed in the battle, and 14,000 were taken captive. Ustadh Sis managed to flee to the mountains, but the general Khazim ibn Khuzayma al-Tamimi followed him and was able to capture him. Ustadh Sis was sent in chains to Al-Mansur, who ordered his execution. Later, Al-Mahdi gave amnesty to 30,000 captives.
- al-Tabari, Muhammad ibn Jarir (1990). The History of al-Tabari Vol. 29: Al-Mansur and al-Mahdi A.D. 763-786/A.H. 146-169. SUNY Press. pp. 44–48. ISBN 9780791401439.