Mujassem is ruled as a disbeliever and the prayer behind him is not valid

“Judge Hussein, may Allah have mercy on him (died 462 AH), mentioned in his commentary on the Mukhtassar of Al-Muzani (Volume 2 / Page 1031), quoting the text of Imam Al-Shafi’i, may Allah have mercy on him, that whoever believes that Allah the Exalted is sitting on the Throne, then such a person is ruled as a disbeliever and the prayer behind him is not valid. End quote.”