Offering Salah While Sitting In Chair Because Of Illness
The principle in the matters of worship is Yusr, i.e. ease. Whenever a person feels hardship in performing a ritual in its standard form, an alternate is provided. This is the principle always keep in mind to know the answer of such questions. for example, Tayammum is prescribed when one is travelling, or sick or find no water. One may run a wet hand on socks when wearing it during wudu, if one has washed his feet before wearing socks. Now come to your question. when one is not able or find difficulty to perform prayer in the normal way, one may perform in any way which is near to the normal form. Using chair to offer prayer while sitting is quite right.