What is the ruling on praying behind an Imam who prostrates only on his forehead without his nose, knowing that he has been alerted but did not respond? What should be done with him, and what is the ruling on past prayers performed behind him?

The majority of scholars hold that prostration on the forehead alone, without the nose, is sufficient, contrary to the Hanbalis who deem it insufficient if one is able to prostrate on both simultaneously.

If a congregant prays behind an imam whose prayer the congregant believes to be invalid, there is a difference of opinion on the matter:

First Scenario: If the congregant does not know that the imam performed an act that invalidates the prayer, the congregant's prayer is valid by agreement of the Salaf and the four Imams.

Second Scenario: If the congregant is certain that the imam performed an act that is impermissible in the congregant's view (such as touching one's private parts or women with lust, or undergoing cupping or phlebotomy, or vomiting, and then praying without ablution), then in this scenario, there is a well-known difference of opinion: First Opinion: The congregant's prayer is not valid, because he believes his imam's prayer to be invalid. Second Opinion: The congregant's prayer is valid. This is the view of the majority of the Salaf and the Maliki school, and it was favored by Shaykh al-Islam Ibn Taymiyyah, who cited the hadith (definition), "They pray for you: if they are correct, it is for you and for them; and if they err, it is for you and against them." He explained that the imam's error does not extend to the congregant, because the congregant believes that what the imam did is permissible for him, and that the imam is either a mujtahid or follows a mujtahid, and that he is not sinful if he does not repeat it. Thus, the prayer of each of them is valid, and each has fulfilled his obligation.