What is the ruling on praying on a chair during obligatory prayers, what are the necessities that permit this, and what is the ruling on standing, bowing, and prostrating in it?

It is permissible for someone who is unable to stand during an obligatory prayer (fard) to pray while sitting on a chair. Scholars have reached a consensus on this matter. The excuse that permits praying while sitting in an obligatory prayer is the inability to stand, or if the worshiper experiences apparent hardship.

Standing, bowing (rukūʿ), and prostrating (sujūd) are pillars of prayer that are not waived in an obligatory prayer when one is capable of performing them. They are waived when one is unable to perform them, according to the Almighty's saying: "Allah does not burden a soul beyond its capacity" (Quran 2:286).

Whoever is unable to perform some of these actions but is able to perform others should omit what they are unable to do and perform what they are able to do. So, if one is unable to stand but is able to perform rukūʿ and sujūd on the ground, they must perform what they are able to. If they are unable to prostrate, they should gesture for it and bring their face as close to the ground as possible.

As for supererogatory prayers (nafila), it is permissible to perform them while sitting, even without an excuse.