To stand upright in Salah is known as Qiyam. Standing up is an obligatory posture of Salah without which the Salah is invalid, unless one cannot perform Salah standing, or is unable to make Sajdah on the ground.
One who is unable to stand and perform Salah due to a genuine medical condition is permitted to sit and perform Salah on the ground and make the Sajdah on the ground.
Should making Sajdah on the ground not be possible, then one is allowed sit on a chair and perform Salah by making isharah (indicating for Ruku and Sajdah).
When sitting on a chair for Salah, Ruku and Sajdah shall be made by lowering the head (indication). The indication for Sajdah should be lower than that of Ruku. If one is performing Salah behind the Imam then the back legs of the chair should be placed in line with the heels of the other musallis in the saff (row) – as illustrated below.
Nabi (Sallallahu Alaihi Wasallam) said: “Perform Salah while standing and if you cannot, then pray while sitting, and if you cannot (sit and perform Salah), then pray while lying on your side.” (Bukhari)
