Answer
(Fatwa: 1082/924=D/1430)
If someone is unable to perform salah with ruku and sajdah standing, he should perform salah sitting whether ruku and sajdah are performed in proper way or with isharah (gesture). So, in case of being able to perform sitting at ground, it is not correct to perform salah sitting in chair. If one is unable to perform salah sitting at ground with proper ruku and sajdah, his salah at chair with isharah is unlawful and the salah will be invalid. Those who perform salah at chair, what excuse they offer, they should explain the same and ask the questions themselves.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband