Rasulullaah (SAW) said, “Perform Salaah while standing up. If you do not have the ability to do this, then perform Salaah while sitting. If you are unable to do even this, then do so while lying on your side.”
[Bukhaari 1117, from Hadhrat Imraan bin Husayn (RA)]
If a person is so ill that he cannot stand, he may perform Salaah with the Ruku and Sajdah while sitting down. If he is unable to perform the Ruku and Sajdah, he may perform them with indications. However, if he cannot even sit, he may perform his Salaah while lying down.