What is the ruling on one who follows a person who has opposed the Messenger (peace and blessings be upon him), disobeyed his commands, altered his Sunnah, and concealed it from Muslims? And what is the ruling on the common people who follow such a person?

Whoever follows other than the Messenger, peace be upon him, is exposed to great danger and must repent immediately. For no one is an example to follow after the Messenger except in what conforms to his law. And whoever turns away from the Sunnah of the Prophet is not of him. Obedience to Allah is linked to obedience to His Messenger. Al-Shafi'i said: "There is no proof in the saying of anyone besides the Messenger of Allah, peace be upon him, no matter how many they are." Ibn Uthaymin clarified that the Imam who is to be followed must strive to follow the Messenger.