All perfect praise be to Allah; and may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.

Leading prayer while sitting on a chair is valid, provided that the Imam is unable to stand up because standing up with the ability to do so is a pillar in obligatory prayer. Allah gave this exemption at times of necessity for He Says {what means}: “Our Lord! Lay not on us a burden greater than we have strength to bear “ {Al-Baqarah/286}; as well as He, The Exalted, Said (What means):"and has imposed no difficulties on you in religion" {Al-Hajj/78}.

Fearing hardship, worsening of sickness or delaying of recovery entitles the sick to pray while sitting

. Al-Imam An-Nawawi {May Allah bless him} said: “ Our fellow scholars said: “ It is not a condition in inability not to be capable of standing up nor experiencing the least hardship; rather, evident hardship is what counts. Therefore, if he feared great harm, worsening of sickness and the like, or drowning- if on a boat- or fainting, then there is no harm in offering prayer while sitting and he isn`t obligated to reoffer it later on.” {Al-Majjmou`4/310}.

As regards the ruling on the prayer of those led by the Imam, it is considered valid because his prayer is considered as such; however, they must pray behind him standing based on the Hadith narrated by A`isha {May Allah be pleased with her}.During the ailment of the Prophet (PBUH) some people came to visits him. He led them in prayer while sitting. but they prayed standing, so he waved to them to sit down. When he had finished the prayer, he said, "An Imam is to be followed, so when he bows, you should bow. and when he raises his head, you should raise yours, and if he prays sitting. you should pray sitting." Abu Abdullah said Al-Humaidi said, (The order of ) "This narration has been abrogated by the last action of the Prophet as he led the prayer sitting, while the people prayed standing behind him. {Bukhari}. Consequently, we follow the last action of the Prophet as he led the prayer sitting, while the people prayed standing behind him. And Allah knows best.