Introduction to Fiqh-us-Sunnah
Whoever has some excuse due to illness and cannot stand during the fard salah is allowed to pray sitting. If he cannot pray in a sitting posture, he may pray while on his side by making gestures. In such a case, his gestures for sajdah should be lower than those for his ruku'. This principle is based on Allah's words: "...And celebrate Allah's praises, standing, sitting, and lying on your sides."
'Imran ibn Hussain says: "I had piles [hemorrhoids], so I asked the Prophet about the prayer and he said: 'Offer the salah while standing and if you cannot do so, pray while sitting, and if you can't do that, then make salah while Iying on your side."' This is related by the group, except for Muslim. An-Nasa'i adds: "And if you cannot offer salah while lying on your side, then do it while lying on your back. Allah does not burden a soul, save with what it can bear."
Jabir reports: "The Messenger of Allah visited a sick person and found him praying on a cushion. The Prophet pushed it aside and said: "Pray on the ground if you can, and if you cannot, then pray by making gestures, and make your sajdah lower than your ruku'.'" This is related by alBaihaqi .
What is meant by inability is that the person if he prays [in the regular way], will suffer hardship, or his disease will aggravate, or his recovery would be hampered, or he will swoon if he prays in the customary manner. One should sit cross-legged while praying in a sitting position.
'Aishah narrates that she saw the Prophet sallallahu alehi wasallam sitting cross-legged while praying. This is related by an-Nasa'i and al-Hakim says it is sahih.: It is also permissible to sit in the manner that one sits while performing the tashahud
One who can offer the salah neither sitting nor standing is to lie down on his side, and if he cannot do that, he is to lie down on his back with his legs toward the qiblah according to his state of health. Ibn al-Munzhir prefers this opinion. On this point, there is a weak hadith reported by 'Ali which states that the Prophet said: "The sick person is to pray standing if he is able. If he cannot do so, he should pray sitting. If he is not able to make the sajdah, he should nod with his head and make the nod of his sajjud lower than that of his ruku'. If he cannot pray in a sitting posture, he should pray while lying down on his right side facing the qiblah. If one cannot pray on his right side, he should pray while lying on his back with his legs stretched out toward the qiblah." This is related by ad-Daraqutni. Some scholars maintain that one can pray in whatever manner is easy for him. It is apparent from the hadith that if one can only nod while lying on his back, then nothing else is obligatory upon him.