Is the prayer of a sick person valid if he prays sitting down, even though he is able to prostrate, for the purpose of a wound healing?

The validity of your prayer while sitting depends on your ability to stand. If you are unable to do so, or find unbearable hardship, or fear harm such as an increase in illness or delayed recovery, then your prayer is valid and you do not need to repeat it. Standing is waived in case of inability or fear of harm, and your reward will not be diminished because you are excused. However, if you pray sitting while you are able to stand without considerable hardship or fear of harm, then your prayer is invalid and you must repeat it.