Wa `alaykum As-Salamu waRahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
It is permissible to offer the prayer on a chair as long as there is a necessity for that.
Answering your question, the General Iftaa’ Department in Jordan, states the following:
Allah, The Exalted, has imposed no difficulties on people in religion, since He said: “… and has imposed no difficulties on you in religion.” (Al-Hajj 22: 78)
In addition, Allah facilitated performing acts of worship in accordance with a person`s capacity because He said: “On no soul doth God Place a burden greater than it can bear.” (Al-Baqarah 2:286).
Moreover, `Imran ibn Husain said :I had a fistula; so I asked the Prophet (peace be upon him). He said: offer prayer in standing condition; if you are unable to do so, then in sitting condition: if you are then at your side (i.e, in lying condition).
In conclusion, necessity knows no law and the state of compulsion is assessed according to the extent of the necessity, so whoever is capable to pray standing, he is neither permitted to sit nor to pray on a chair.
However, it is worth pointing out that in the past it wasn’t common for Muslims to pray on chairs. Nonetheless, this is allowed, nowadays, but in line with the degree of necessity.
However, the one who prays on a chair shouldn’t pray in the middle of the row in order not to harm or disturb the people praying behind him. Rather, he should pray in the sides of the row or in the back rows.
Almighty Allah knows best.