Firstly: Those who Cannot Stand in Prayer
Whoever cannot stand in prayer – such a sick person or the likes – is permitted to pray sitting and is not required to make up the prayer thus offered.  He may also pray on a chair if it is gentler on him in which case he bows and prostrates in the air, causing his prostration to be lower than his bowing if he cannot prostrate on the ground. Consensus on this was related by Ibn al-Mundhir, Ibn Rushd, al-Nawawi, and Ibn Taymiyyah.
Secondly: How a Sick Person Sits in Prayer
Whoever prays sitting due to an illness may sit in any manner possible: he may sit cross-legged or in iftirash.  They differ regarding which is superior. The Maliki and Hanbali schools, as well as one position among Shafi`is hold that sitting cross-legged is superior. This is the position of choice of Ibn Baz and Ibn `Uthaymin. Shafi`is hold that it is superior to sit in iftirash like someone sitting for the first tashahhud. This is by agreement of the four schools of jurisprudence.
Thirdly: How a Sick Person Sits for Tashahhud
If a sick person sits instead of standing, then he sits during tashahhud normally. Consensus on this was related by al-Kasani and Ibn Nujaym.
Fourthly: Lying on One’s Side
It is permissible for a sick person to pray on his side if unable to sit. This is by agreement of the four schools of jurisprudence.
Fifthly: One Unable to Bow or Prostrate
A sick person who is unable to bow or prostrate signals  Ibn al-Atheer says: “Signalling means pointing with body parts, like the head, hands, eyes, and eyebrows. Here specifically, he means with the head.” (Al-Nihayah, 1/81) for them, with prostrating being lower than bowing. This is by agreement of the four schools of jurisprudence. It is the position of most scholars. Ibn Rushd and Ibn Taymiyyah related consensus on the non-requirement of bowing and prostrating for one not able to do so.
Sixthly: The Ruling of the Sick Praying in Congregation
A sick person may abandon congregational prayers. Consensus on this was related by Ibn al-Mundhir and Ibn Hazm. Ibn Qudamah ascribed this position to the generality of scholars.
When Illness Permits Abandoning the Congregational Prayer
The existence of an illness permits abandoning the congregational prayer when it is difficult to attend the congregation.  Al-Nawawi says: “If it is a slight illness that does not crush one’s determination, like a sore molar, a slight headache, or a mild fever, then this is not a valid reason. They indicate that it (the illness) must cause the same hardship as walking in the rain.” (Al-Majmu`, 4/205)
Seventhly: Combining Prayers Due to Illness
It is permissible to combine prayers due to illness.  The sign of an illness according to Malikis is that one fear losing consciousness, fever, or the like. As for the the definition of some Shafi`is, it is everything that makes offering every obligation during its allotted time difficult, like the difficulty of walking in the rain such that one’s clothing gets wet. It is also said that there must be additional, obvious hardship, such as that which permits sitting in prayer. As for the Hanbalis, an illness that permits combining prayers, it is one that brings about difficulty and weakness when offering every prayer on time. This is the position of the Maliki  Malikis hold that combining prayers due to illness can only be done at the time of the first prayer. If a sick person gets better, he repeats the second prayer during its normal time. and Hanbali schools, a group of Shafi`is, and the position of choice of al-Nawawi, Ibn Taymiyyah, al-Shawkani, Ibn Baz, and Ibn `Uthaymin.