Ustadh Sis

Ustadh Sis (also spelled: Ustad Sis, or Ostad Sis, Persian: استاد سیس) was a Persian heresiarch and anti-Abbasid rebel leader. It is speculated that he was once a governor of Khorasan and possibly father to Al-Ma'mun’s Iranian mother, Marajil, which would make him Ma'mūn's maternal grandfather.

Based in the eastern fringe of Khorasan, in the mid-8th-century he claimed he was a prophet of God and managed to gain followers among the villagers in that area. Many were previously followers of Bihafarid, whom the Abbasid commander, Abu Muslim, had crushed militarily.