The Grand Ayatollah Khamenei answered questions about the incorrect recitation by the prayer leader in the congregational prayer.
Question: The recitation of some prayer leaders is incorrect (Arabic pronunciation). Can a person who is able to recite correctly follow them? Some say you can perform Salaat in congregation but later on perform the same Salaat on your own, I do not have the time to do so, what is my duty? Can I take part in congregational prayers but recite Surah Al-Fatihah and Surah Al-Ikhlas quietly?
The Grand Ayatollah Khamenei: If the recitation by prayer leader is incorrect, following him and his congregational prayer is void (Baatel) and if it is not possible to re-perform the same Salaat, then there is no problem in not following the pray leader. But reciting quietly in Salaats which need to be recited loud is not correct for following the prayer leader and is not separate.
Source: PERSIAN SHAFAQNA