One of the distinguishing features of Islam is easiness. Allah Almighty says:(He hath chosen you and hath not laid upon you in religion any hardship) (Al-Hajj 22:78). Thus, if a person doesn’t have the ability to pray standing due to a certain illness, he is permitted to pray while sitting on a chair.
The Islamic Religious Council of Singapore, issued the following: The one performing the Prayer must abide by its rules and regulations. Although it is allowable for someone who is unable to stand to perform the Prayer sitting down, or for someone unable to sit to perform the Prayer lying down, still, we must know when that is allowable.
Praying by sitting down on a chair is only permissible for someone not able to perform ruku` or sujud due to physical inabilities. But if he is able to stand, he is not allowed to perform the Prayer while sitting down.