Kitab Al-Salaah (The book of Prayer)

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Performance
Of The Prayer By One Who Is Sick

           
A sick person has to perform the prayer in accordance with the person's
conditions. The Prophet (peace be upon him) was once asked by a sick
companion as to how he can perform the prayer and the Prophet (peace be
upon him) replied him as follows: “Pray standing but if unable
then by sitting and if still unable then do so whilst on your back”
(Reported by Imam Bukhari).

           
Thus:

• If, it is impossible for the sick person to stand up, he/she
  shall perform the prayer by sitting and making token gestures for the
  "Rukuu" and "Sujuud" positions by bringing the head slightly down as
  indication for "Rukuu" and slightly further down as indication for
  "Sujuud".
• If, it is impossible for the sick person to pray sitting,
  he/she shall perform the prayer by lying on his/her back. Allah
  (subhanahu wa ta'ala) says: “Those are they (men of
  understanding) who remember allah while standing, sitting and reclining”
  (Qur'an 3:190-191). The person's legs are directed towards the Qiblah
  and will make mere token gestures as far as possible with the head for
  the bowing (Rukuu) and the prostration (Sujuud) positions.
• If, still it is impossible for the sick person to perform
  the prayer while lying as discussed in No. 2 above, then, the person
  shall perform the prayer by heart. The person shall come up with all
  the required recitations as far as possible and will recall to mind all
  the necessary postures for the prayer.

Such is the importance of the Prayer in Islam that a person must
perform it no matter what his/her condition.