Is it permissible to pray alone and refrain from praying behind an imam who is unable to prostrate or sit between the two prostrations and for the tashahhud, so he prays standing and then sits on a chair for prostration and tashahhud, while the principle is that if the imam prays sitting, the congregants also pray sitting?

Scholars have differed concerning the ruling on a sick person leading the prayer, or one who is unable to stand. The correct view is that it is permissible for him to lead the prayer and that following him is valid. This is supported by the hadith of Abu Hurayrah (may Allah be pleased with him): "The Imam is appointed to be followed, so do not differ from him; when he bows, bow; when he says, 'Allah hears him who praises Him,' say, 'Our Lord, to You be praise'; when he prostrates, prostrate; and if he prays sitting, then all of you pray sitting." This ruling applies to one unable to bow, prostrate, or sit; prayer behind him is valid, but the able congregant performs the pillars completely (he bows and prostrates with full prostration). As for the opinion of those who do not pray behind a disabled Imam, it is the view of some scholars, but the more correct view is to pray behind him.