The basic principle concerning the prayer of one who is sick is that he should do whatever he is able to of the essential parts of the prayer, and he does not have to do what he is unable to do. This is indicated by a great deal of evidence from the Qur’aan and Sunnah. Allaah says (interpretation of the meaning): 

“So keep your duty to Allaah and fear Him as much as you can”

[al-Taghaabun 64:16] 

“Allaah burdens not a person beyond his scope”

[al-Baqarah 2:286]

And the Prophet (peace and blessings of Allaah be upon him) said: “If I command you to do a thing, then do as much of it as you can.” Narrated by al-Bukhaari, 7288; Muslim, 1337. 

It was narrated that ‘Imraan ibn Husayn (may Allaah be pleased with him) said: “I was suffering from haemorrhoids and I asked the Prophet (peace and blessings of Allaah be upon him) about praying. He said, ‘Pray standing, and if you cannot, then sitting down, and if you cannot, then lying on your side.’” Narrated by al-Bukhaari, 1117. 

Based on the above, if you are able to pray standing up then you have to stand. Then if you become unable to stand or it is too difficult for you to stand, then you can sit down whilst praying. 

It is permissible to sit on a chair or on the ground, depending on what you are able to do and what is easy for you. But it is better to sit on the ground, because the Sunnah is for a person to sit cross-legged in the place where one would stand and bow, and this is not easy to do on a chair.  

Shaykh Ibn ‘Uthaymeen said:  

If a person cannot pray standing, he should pray sitting, but it is better to sit cross-legged in the place of standing and bowing. 

From his essay, Tahaarat al-Mareed wa Salaatuhu 

This sitting cross-legged is not obligatory; he may sit however he wants because the Prophet (peace and blessings of Allaah be upon him) said: “If he cannot then sitting,” and he did not explain how he should sit. 

See al-Sharh al-Mumti’, 4/462. 

If it is too difficult for you to prostrate and bow, then you can lean forward and make the leaning for sujood deeper than that for rukoo’. 

If you can stand, then lean forward for rukoo’ whilst standing, and lean forward for sujood whilst sitting, then you should do that, because standing is closer to bowing than sitting, and sitting is closer to sujood than standing. 

Shaykh Ibn Baaz said: 

Whoever is able to stand but is unable to bow or prostrate is not relieved of the obligation to stand, rather he should pray standing up, then lean forward for rukoo’ (i.e., whilst standing) then sit and lean forward for sujood… and he should make leaning for sujood deeper than that for rukoo’. If he is only unable to prostrate then he should do rukoo’ and then lean forward for sujood. 

If during the prayer the sick person becomes able to do something that he was unable to do, such as standing, sitting, bowing or prostrating, he should start to do that and continue from whatever he has already completed of his prayer. 

From his essay Ahkaam Salaat al-Mareed wa Tahaaratihi. 

Shaykh Ibn ‘Uthaymeen said: 

Whoever is not able to bow should lean forward whilst standing, and whoever is not able to prostrate should lean forward whilst sitting. 

Al-Sharh al-Mumti’, 4/475 

And Allaah knows best.