All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Whoever is unable to stand during the prayer, or is unable to bow or prostrate due to a sickness, should perform the prayer according to his ability as Allaah Says (what means): {So, keep your duty to Allaah and fear Him as much as you can.}[Quran 64:16]
Therefore, if one is not able to perform the prayer while standing, he may perform the prayer while sitting. If he is able to stand during the prayer but is unable to bow or prostrate, then he may lean with his head for bowing while in the standing position, and he may lean with his head for the first prostration while in the standing position as well because the first prostration is usually done from a stand up position after rising from the bowing position. Then he may sit down and lean with his head while sitting (on a chair for instance) for the second prostration as this prostration is usually made from the sitting position of the first prostration. This is the preponderant opinion, Allaah Willing.
For more benefit, please refer to Fatwa 88501.