What is the ruling on the prayer of one who follows an imam who does not pronounce the letter "س" (seen) well in Al-Fatiha, such as saying "wa 'iyyaka nata'een" instead of "wa 'iyyaka nasta'een," or "al-rat al-mutqim" instead of "al-sirat al-mustaqim"? And is it obligatory to repeat the prayer, or the audible or silent rak'ahs? And what is the ruling on the prayer of one who follows an imam who pronounces "al-sirat al-mustaqeem" instead of "al-sirat al-mustaqim"?

If a melodic error (laḥn) in Al-Fatiha involves omitting a letter or replacing it with another, it invalidates the prayer (ṣalāh). It is not permissible to pray behind an imam who makes such errors if he is capable of learning to correct them. However, if the melodic error comes from someone who is unable to learn, such as a person with a lisp, some scholars have deemed prayer behind him valid. If you are certain that the imam has omitted a letter from Al-Fatiha, you must prompt him to correct his recitation; otherwise, his prayer will be invalid, and prayer behind him will not be sound. However, if there is no certainty about the omission of a letter, then prayer behind him is valid, because the default assumption is the correctness of Muslims' prayers. One must be cautious of obsessive doubts (waswasah) in this matter.