How is it forbidden for one who is sitting to lead one who is standing in prayer, even though the Prophet, may Allah bless him and grant him peace, led the people in prayer while he was sitting?

Scholars have differed on the issue of an imam who is sitting leading those capable of standing in prayer. The Malikis and those who agreed with them held that it is not valid, because those capable of standing would be neglecting the obligation of standing, and the imam does not bear this on their behalf. The Hanbalis made an exception for the imam of a neighborhood whose ailment is expected to be temporary. Conversely, the Hanafis and Shafi'is permitted it. The preponderant opinion is that of the Hanbalis, and the action of the Prophet, peace and blessings be upon him, is interpreted in light of this, as he led the Companions in prayer while sitting due to an ailment that was expected to be temporary.