"From Imran bin Husain who said: 'I asked Allah's Messenger (S) about Salat for a sick person. He said: "He performs Salat standing, if he is not able then sitting, if he is not able then on his side."
Jami` at-Tirmidhi Book 2, Hadith 224
← Previous Hadith "I asked Allah's Messenger (S) about the Salat for a man who is sitting. He said: 'Whoever performs Salat while standing then that is more virtuous. And whoever performs Salat while sitting, then he gets half the rewards of the one standing, and whoever performs Salat while lying down, then he gets half the rewards of the one sitting.'"

Jami` at-Tirmidhi Book 2, Hadith 223 Next Hadith → "I did not see Allah's Messenger (S) praying voluntary prayers sitting until the year before he died. Then he would perform Salat for the voluntary prayers sitting, and he would recite a Surah and prolong it such that it would be longer than the longest of them."

Jami` at-Tirmidhi Book 2, Hadith 225