Answered by: Imam Abdul-Malik Sheikh
Praying Salah on a chair
بِسْمِ اللهِ الرَّحْمنِ الرَّحِيْم
In the name of Allah, the Most Gracious, the Most Merciful
If a person is unable to stand due to illness or physical disability and fears that by doing so his or her condition will get worse, then he or she can perform the Salah sitting down.
‘Imrān ibn Husayn (may Allāh be pleased with him) said: I had hemorrhoids so I asked the Prophet (peace and blessings of Allah be upon him) about praying. He said: “Pray standing up; if you cannot, then pray sitting down; and if you cannot, then pray (lying) on your side.” (Reported by al-Bukhāri)
This is because the obligations are to be fulfilled according to people’s abilities.
Whoever starts praying the prayer seated, and then recovers from his or her illness, will complete the remainder of his prayer standing.
It should be noted that if a person is able to stand and pray with some difficulty, they should not leave the standing. For example, if they are able to stand for the duration of the Takbir Tahrima they should do so and then sit down.
Only Allah knows best
Written by Imam Abdul-Malik Sheikh
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham
 Al-Ikhtiyār Li-Ta’līl al-Mukhtār Vol. 1 p. 102
 Al-Lubāb Fi Sharh al-Kitāb of Imām al-Mīdāni vol. 1 p 80