The Prayer of the ill (mareed)

 

Pray standing, if you cannot, say it sitting, if you cannot, say it lying on your side
  [Sahih al-Bukari]

Salah is the most important pillar after the Shahadah, thus it can't be neglected, nor will a believer be excused from observing it. We shall all be questioned on our observance of it, and it will be the first thing asked about on the Day of Judgement. Even if a person is ill he should continue to observe Salah/namaz.
Intellectual Copyright (c) 2003 Mutmainaa. All Rights Reserved. Do not copy and paste without prior permission. www.oocities.org/mutmainaa However, if due to one's condition one cannot observe the basic elements of Salah, one is licensed to pray sitting if he cannot stand, or on his side if he cannot sit, or lying on his back, if he cannot say it lying on his side.

Still, if he cannot make Ruku' or Sujud, he should sign with his head, but must sign his Sujuud lower than his Ruku'.

"Men who celebrate the praises of Allah, standing, sitting and lying down on their sides

[surah ali Imran; 3:191]

The Prophet (salAllahu alayhi wasalam) told Imran bin Husain, who had complained of illness:

"Pray standing, if you cannot, say it sitting, if you cannot, say it lying on your side."
[Sahih al-Bukari]

To sit for Salah in this situation, one should sit Mutarabi'an, with the legs crossed, according to the Sunnah of the Prophet. However, sitting, as in the position for Tashahud, is all right if one cannot sit Mutarabi'an.