The viewpoint of Ayatullah Mahdi Hadavi Tehrani is as follows:

1- According to an obligatory precaution, it is necessary to observe the correct I’rabs (vowels) during the prayer.

2- Correct recitation of surah Fatihah and the ensuing surah is a condition of jama’ah prayer (not any of the other things that are recited during the prayer like the phrases in ruku’ and sujud), therefore in the case mentioned above, since the imam is incorrectly reciting the بسم الله الرحمن الرحیم , which is part of surah Fatihah and the other surah, as an obligatory precaution, it isn't permissible to follow him.

3- In the case mentioned above, it isn't permissible to follow the imam. In the case of one unknowingly following the imam, his prayer is valid (if he finds out after the prayer, and in the case of finding out in the middle of the prayer, he needs to continue the prayer in furada and by himself, picking up from where the imam incorrectly recited and going on from there and his prayer will be correct).

4- If a ma’mum (one who follows the imam in prayer) is a person who excessively doubts about whether something has been recited correctly or not, he shouldn’t pay attention to his doubt and must assume that the imam’s recitation is correct.

For further information on the viewpoints of different scholars regarding this subject, see:

1- The duty of the ma’mum when the imam recites the phrases of the prayer incorrectly, Question 3576 (website: 4151).

2- Following an imam of jama’ah whose recitation is incorrect, Question 3549 (website: 3801).

3- The duty of the ma’mum when the imam makes a mistake, Question 391 (website: 407).