Offering Salah behind an Imam suffering from a disability

Q: Next to my house is a Masjid (mosque) whose Imam (the one who leads congregational Prayer) prays while sitting on a chair and using a table as he is unable to make Ruku` (bowing) or Sujud (prostration) on the floor because of a pain in his knees. It may be worth mentioning that some of the Ma'mums (persons being led by an Imam in Prayer) who pray behind the concerned Imam know some parts of the Qur'an by heart. Is it thus permissible for me to perform Salah (Prayer) behind him?

A: In case that such an Imam is permanently unable to stand for Salah and make Sujud on the floor, Salah which is performed behind him by Ma'mums who are physically fit is invalid. Moreover, it is impermissible for the concerned Imam to lead the Salah as he is unable to perform some of its Rukns (integrals). Consequently, the Imam in question has to be substituted by another one who is able to perform all the Rukns of Salah.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.