Reference: http://www.alfawzan.af.org.sa/node/16883
Question: If the doctor is of the opinion that the sick person should pray all his prayers sitting down, or seated on a chair, is he then permitted to do so? And will he attain reward [for doing so]?
Response: Yes, if the doctor prescribed that he should pray sitting down [or on a chair], then he is not required to stand, as he is excused from standing. That is because the doctor has prescribed this for him; and the doctor is of the opinion if he were to stand for prayer, then it would be harmful for him, or intensify his illness, in which case he should pray sitting down [or on a chair]. This is based upon his ﷺ saying:
«The sick should pray standing, and if he is unable, then sitting down, and if he is unable, then lying on his side.» – transmitted by al-Bukhaaree
