Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.120A.

One must stand during the prayer, if at all possible. Says Allah, “Guard and preserve the prayers and the mid-most prayer, and stand for Allah with devotion.” Reported ‘Umar ibn Hussain, “I had some physical problem, so I asked the Prophet, upon whom be peace, about the prayer, and he said, ‘Pray standing; if you are not able to; pray sitting, if you are not able to; pray (while lying) on your side.” (Related by al-Bukhari.) Most scholars say that one should not put his feet together while standing in prayer.

For voluntary prayers, one can pray sitting even if he can stand, but one who stands receives a larger reward than one who sits. ‘Abdullah ibn ‘Umar related that the Prophet, upon whom be peace, said, “The prayer of one who sits is half of the prayer.” (Related by al-Bukhari and Muslim.)

If one can not stand, he may pray according to what he is capable of doing, as Allah does not burden a soul beyond its ability. He will get a complete reward for the prayer. Abu Musa reported that the Prophet said, “If a slave (of Allah) is sick or travels, he will get a reward for those acts similar to what he would get if he was healthy and at home.”