'imran ibn husain (radiallaahu 'anhu) said, "i was suffering from haemorrhoids (piles), so i asked the messenger of allaah (sallallaahu 'alaihi wa sallam) and he said, pray standing; if you are not able, then sitting down; if you are not able to do so, then pray lying down.17
'imraan ibn husain also said, "i asked him (sallallaahu 'alaihi wa sallam) about the prayer of a man while sitting, so he said: he who prays standing, that is better; he who prays sitting, his reward is half that of the former. he who prays lying down (and in another narration: reclining), has half the reward of the one who sits.18
this applies to the sick person, for anas (radiallaahu 'anhu) said, "the messenger of allaah (sallallaahu 'alaihi wa sallam) came out to the people while they were praying sitting due to illness, so he said: verily, the prayer of one who sits is (worth) half of the prayer of the one who stands.19
once "he (sallallaahu 'alaihi wa sallam) visited a sick person and saw him praying (leaning) on a pillow, so he took it and cast it aside. so the man took a stick to pray (leaning) on it, but he took it and cast it aside and said: pray on the ground if you can, but otherwise make movements with your head, making your sujood lower than your rukoo."20
17 bukhaari, abu daawood and ahmad.
18 ibid. khattaabi said, "the meaning of `imran's hadeeth is intended for a sick person who is able to undergo hardship and stand with difficulty. hence the reward of praying sitting has been made half of the reward of praying standing: encouraging him to pray standing while allowing him to sit." ibn hajr said in fath al-baari (2/468): "this deduction is valid".
19 ahmad & ibn maajah with a saheeh sanad.
20 tabaraani, bazzaar, ibn as-samaak in his hadeeth book (67/2) & baihaqi . it has a saheeh isnaad as i have explained in silsilah al-ahaadeeth as-saheehah (323).