Answer

Qiyām (قِيَام) is a Farḍ (فَرْض) in Farā’iḍ (فَرَائِض), Wājibāt (وَاجِبَات) and Sunan (سُنَن) of the Fajr (فَجْر) only. If these prayers are offered sitting without a valid Sharʿī (شَرْعِي) excuse, they will not be established. If one can’t stand by themselves, but can take assistance from a stick, or a wall, then he must do so and stand as much as he can.

One of the reason when Qiyām (قِيَام) can be left is when the person is unable to do Sujūd (سُجُود) on the floor or something raised above the ground up to 12 fingers (or approximately 9 inches). Then this inability to do Sajdah-e-Ḥaqīqī (سَجْدَة حَقِيقِي) removes the obligation of Qiyām (قِيَام) as well.

Thus, if a person can’t do Qiyām (قِيَام) or can do it partially then he is allowed to sit in prayer only for the length of the time he is unable to do qiyaam. But if he is still able to do Sajdah (سَجْدَة) and Rukūʿ (رُكُوع) properly, then he must do that even if it means raising something above the ground up to 9 inches. Simply gesturing for Rukūʿ (رُكُوع) and Sujūd (سُجُود) while sitting on a chair will not be sufficient in this case.

If that person is able to do Sujūd (سُجُود) on the floor, then there is no reason for him to sit on the chair. Thus, sitting on a chair for someone able to do Sajdah (سَجْدَة) on the floor isn’t permissible.

If he can’t do Sujūd (سُجُود) on the floor then the obligation of Qiyām (قِيَام), even if he can stand, will be eliminated and he can perform the entire Ṣalāh (صَلَاة) while sitting, ideally on floor , but also on on a chair if that is more convenient.

For Tarāwīḥ (تَرَاوِيح), if one finds it difficult to pray while standing, or there is a genuine fear that their illness may worsen or recovery may be delayed, then there is no harm in praying while sitting.

However, if there is no valid excuse for not standing, then praying Tarāwīḥ (تَرَاوِيح) while sitting will still count as having performed the prayer, but it is Makrūh Tanzīhī (مَكْرُوه تَنْزِيهِي) (a disliked action). In such a case, the person who prays sitting without excuse will receive half the reward compared to the one who prays standing.

Furthermore, according to some scholars, if Tarāwīḥ (تَرَاوِيح) is performed while sitting without a valid excuse, it may not even be considered valid. Therefore, greater caution is required in this matter. If there is no genuine excuse, Tarāwīḥ (تَرَاوِيح) should be performed while standing.

If a person is able to do Rukūʿ (رُكُوع) and Sujūd (سُجُود), and performs all prayers standing with normal prostration and bending, and sits on a chair for part of the Tarāwīḥ (تَرَاوِيح) then it will not be permissble.