God says
.…And if you are in the state of requiring total ablution bath, purify yourself. But if you are ill, or are traveling, or have just satisfied a want of nature, or cohabited with a woman, and can find no water-- then take resort to pure dust, passing therewith lightly over your face and your hands (Tayammum). God does not want to impose any hardship on you, but wants to make you pure, and to bestow upon you the full measure of His blessing, so that you may cause to be grateful.
Prophet said
“Pray standing; if you are unable (to do it) then sitting; if (that too is) not possible then (lying) upon a side’ -- H: Bukhari. N: Imran-b-Husian
If a sick person cannot perform Ruk’u or Sajdah and cannot bend much then he should say prayers sitting nodding head a little more for Sajdah than for Ruk’u. One need not put pillow or other things in front to perform prostration (upon it). If he can’t sit up he can lie on with a pillow and keep his knees up; if he can’t do this then he should spread his legs towards Qibla and say prayers by signs and nodding, doing a little more for Sajdah than for Ruk’u. If he can’t say prayers even with signs and nodding, he can defer the prayers till he regains strength. If he does not gain strength within 24 hours or more, it is only then that the lost prayers are condoned as also in the case of losing consciousness for 24 hours or more.
(Handy Reference Book on Islam- Page 194)
