Shaykh Saalih al-Fowzaan (may Allaah preserve him) was asked about how one is to pray in a chair when needed.
The reply: Prayer is the second pillar of Islaam. It is the foundational support-post of Islaam, and it is the first of all deeds a person will be held to account for on the Day of Judgment. If it is accepted [by Allaah], the rest of his deeds will be accepted. If it is rejected, the rest of his deeds will likewise be rejected. It is an obligation that no Muslim is ever excused from so long as he remains of sound mind. A Muslim prays according to his ability.
As Allaah has said, “Fear Allaah to the best of your ability.” And He, the Most High, has said, “Allaah does not burden any soul beyond its scope.” Furthermore, the Prophet, may Allaah raise his rank and grant him peace, said, “When I have forbidden you from something, stay away from it [entirely]. When I have ordered you to do something, do as much of it as you are able.”
From this [generality] is prayer. A Muslim is to pray to the best of his ability, the best he can, due to the statement of the Prophet, may Allaah raise his rank and grant him peace, “A sick person prays standing. If he is unable, then he may pray sitting. If he is unable, then he may pray [laying] on his side.” In one narration, “If he is unable, then laying back with his feet toward the qiblah.”
Prayer has its required conditions (shuroot), necessary elements (arkaan, lit. pillars), obligations (waajibaat), and recommended manners (sunan). A sick person is to do as much of these things as he is able to. Some things are to be done while standing, while others are done while sitting or in prostration.
What is done standing is the initial takbeerah (saying, “Allaahu akbar”), reading [Soorah] Al-Faatihah and whatever else is easy of the Qur’aan, and bowing (rukoo’), which is done by bending over with one’s head and back until the hands reach the knees, saying in that position, “Glorified be Allaah, the Great One.”
What is done while sitting is the tashahhud, the prostration, saying, “Glorified be Allaah, the Most High,” and the tasleem.
How does a [sick] person perform these actions?
If he is able to stand and sit, but cannot bow or prostrate, then he motions with his head for the bowing while he stands, and he motions with his head for the prostration while he sits.
If he is able to sit, but cannot stand, then he performs the initial takbeerah, the recitation of Al-Faatihah and what is easy of the Quran to recite [while sitting]. He motions with his head for the rukoo’ (bowing) while he sits. He prostrates on the ground if his able. If not, then he motions with his head for the prostration while sitting, making his nodding [or bending over] lower for prostration than for bowing.
When praying seated, it is better and more complete to sit on the floor if one is able. If not, then he may sit in a chair, and he does everything a person would do sitting on the floor.
The chair is to be lined up along with the row, and it should be a small chairwhenever possible, so it does not take a large amount of space and encroach upon the space of those next to, behind, and in front of him.
He does not pray directly behind the imaam, to leave that place for healthy people who could take his place when needed, or prompt him when he cannot remember something in his recitation. This is due to his statement, “Let the people of intellect and understanding stand directly behind me [in congregational prayer].”
This was written by [Shaykh] Saalih ibn Fowzaan al-Fowzaan, Member of the Council of Senior Scholars, on 08-17-1433.
Translation and footnotes: Moosaa Richardson
 With the exception of women during their menses and post-natal bleeding. (All footnotes are from the translator.)
 Soorah at-Taghaabun (64:16)
 Soorah al-Baqarah (2:286)
 From an authentic hadeeth collected by Al-Bukhaaree (no.7288) and Muslim (no.1337).
 From the meaning of an authentic hadeeth collected by Al-Bukhaaree (no.1117).
 This additional wording is not authentic as a hadeeth. Review: Irwaa’ al-Ghaleel (no.558).
 From an authentic hadeeth collected by Muslim (no.432).