Islamic Ruling on Praying Behind a Sitting Imam
If the imam prays sitting due to an excuse, the followers must pray standing if they are able, and praying sitting without an excuse is not valid.
According to the hadith narrated by Aisha رضي الله عنها, during the illness of the Prophet ﷺ, Abu Bakr رضي الله عنه first led the prayer. Later, when the Prophet ﷺ felt better, he came to the mosque with support and led the prayer while sitting.
Islamic scholars used this narration to explain the Islamic ruling. They state that it is allowed for an imam to lead the prayer while sitting if he has a valid excuse. However, if the followers are able to stand, it is compulsory for them to pray standing.
The fatwa of Islamic scholars is clear. If a follower prays sitting while he is able to stand, his prayer will not be valid. Therefore, in such a case, the imam prays sitting due to his excuse, and the followers pray standing if they have the ability.
This Islamic ruling is based directly on the mentioned hadith and the explanation of the scholars.