The Messenger of Allah (may Allah's peace and blessings be upon him) was once riding a horse and fell down from it, injuring his right side

The Prophet (may Allah's peace and blessings be upon him) was riding a horse and fell down from it, injuring his right side. So he led the Companions in one of the prayers while he was sitting. In turn, they prayed behind him while they were sitting. When he finished the prayer, the Prophet (may Allah's peace and blessings be upon him) informed them that the congregation follows the Imām in everything. When he says "Allah is the Greatest," they say it as well; when he bows, they bow; when he prostrates, they prostrate; if he prays standing, then they should pray standing like him; and if he prays sitting, then they should pray sitting like him. This is so if he starts the prayer in the sitting posture and is a regular Imām. And this is what happened with the Companions (may Allah be pleased with him) when the Prophet (may Allah's peace and blessings be upon him) fell from the back of his riding mount one day and his right side was affected, and so he prayed while sitting, and the Companions prayed behind him while sitting too.