What is the ruling on correcting the Imam in congregational prayer, who is permitted to do so, and is the prayer invalidated if he is corrected by someone who is not one of the congregants?

The congregant must prompt his Imam if the Imam pauses or makes a mistake in Al-Fatiha. As for other than Al-Fatiha, there are three scenarios:

1. A congregant prompts him: This is permissible and his prayer is valid.

2. Someone who is not a congregant and is not praying prompts him: This is permissible and does not invalidate the Imam's prayer according to the Malikis and Hanbalis, but it does invalidate it according to the Hanafis.

3. Someone who is not a congregant but is praying prompts him: This is disliked and does not invalidate the prayer according to the Hanbalis. The Malikis differ on whether the prompter's prayer is invalidated without affecting the prayer of the one being prompted, while both prayers are invalidated according to the Hanafis.