What is the ruling on praying behind an Imam who makes mistakes in reciting Al-Fatiha or the takbirat, and what should we do if we discover his mistake during the prayer, and is it obligatory to repeat the prayers that are found to have been invalid?

If the worshipper (ma'mum) does not know that the Imam makes a grammatical error (lahn) in Al-Fatiha that distorts the meaning, then his prayer is valid. He is not required to investigate the Imam's recitation, because it is presumed that whoever leads the prayer recites well. However, if he hears the Imam's recitation and knows that he makes a grammatical error that distorts the meaning, then he must separate from the Imam and complete his prayer. It is not permissible for him to follow the Imam after becoming aware of his condition. If he follows him, his prayer is invalid and he must repeat it. This applies if the worshipper recites Al-Fatiha well. But if he is like the Imam in making errors, their prayer is valid, and both must learn. The Malikis hold that an unintentional grammatical error does not invalidate the prayer. The Hanafis consider it valid to follow someone who substitutes a letter in Al-Fatiha due to widespread affliction (umum al-balwa).