Under what conditions may a seated person who is capable of standing lead the prayer?
Chapter on Imamate and Congregational Prayer
Al-Mughni
Book of Prayer
Primary text
A seated person capable of standing may not lead the prayer except under two conditions. First, he must be the established Imam of the locality (*Imam al-Hayy*). Ahmad stipulated this because the congregation has no need to appoint someone who misses a pillar if he is not the regular Imam; one should not cause the omission of a pillar without need, and the Prophet (peace be upon him) was the regular Imam when he prayed seated. Second, his illness must be one from which recovery is hoped for, as appointing someone chronically unable to stand as the regular Imam leads to the permanent abandonment of standing when there is no necessity. The precedent is the action of the Prophet (peace be upon him), whose recovery was anticipated.
Supporting text
If the Imam started the prayer standing but became ill and sat down, the followers continue standing if Abu Bakr was leading when the Prophet joined seated. This is because standing is the original state, and whoever begins the prayer standing must complete it while standing if able, analogous to the ruling on completing a prayer in a place where shortening was initially permissible.