What is the ruling on one who leaves congregational prayer behind a specific imam, claiming that he prolongs the prayer, then exits the mosque and is followed by some of the worshipers?

The Imam should lighten his prayer out of consideration for the conditions of the worshippers, for the Prophet, peace and blessings be upon him, said: "If one of you leads the people in prayer, he should lighten it, for among them are the young, the old, the weak, and the sick." If the Imam prolongs the prayer to an extent that harms the people, he should first be gently and wisely asked to lighten it, and what has been narrated concerning lightening should be explained to him.

If he responds, then it is not permissible for anyone to leave after the Iqama except for an excuse. If he does not respond, then it is permissible for a person to leave and pray in another congregation if one is available, otherwise he should pray alone; and his prayer is valid. The proof for this is the story of Mu'adh ibn Jabal when he prolonged the prayer, and the Prophet, peace and blessings be upon him, said: "Are you a tempter, O Mu'adh?"

Nevertheless, it is better to be patient and pray with the mosque's congregation to preserve the unity of the community and avoid division and discord, and out of consideration for the Imam's feelings, for the Prophet, peace and blessings be upon him, said: "The hand of Allah is with the congregation."