What is the ruling on the prayer of someone who prays the obligatory (fard) prayer or Tarawih on a chair in the mosque with the Imam?

Standing in obligatory prayers is a pillar, and the prayer becomes void if it is omitted without an excuse, due to the Almighty's saying: "And stand before Allah, devoutly obedient," and the Hadith: "Pray standing." As for supererogatory prayers, it is permissible to sit even with the ability to stand, and the reward for the one who sits is half the reward of the one who stands, unless he is unable to stand, in which case he receives the same reward as the one who stands. It is not permissible to sit on a chair in obligatory prayers for one who is able to stand, unless there is hardship that causes harm.