The Ruling on praying behind an Imam Who speaks ungrammatically
Question :
There is an Imam who speaks ungrammatically when reciting the Qur'an and sometimes adds or omits letters from Qur'anic Verses. What is the ruling on prayer behind him?
Answer:
If his ungrammatical speech does not change the meaning, there is no objection to prayer behind him, such as saying: "Rabba" or "Rabbu" in the Verse: "Al-hamdu lillahi Rabbil-'alameen. (All praise and thanks be to Allah, the Lord of the worlds)." Likewise, saying: "Ar-Rahmana" or "Ar-Rahmanu" and the like.
However, if it changes the meaning, then you should not pray behind him if he does not benefit from teaching or being corrected, such as reciting: "Iyak" instead of "Tyyaka" or reciting: "An'amti" or "An'amtu" instead of "An'amta". If he accepts learning and improves his recitation upon being corrected, his prayer and his recitation will be correct.
It is prescribed in all circumstances for the Muslim to teach his brother - in prayer and outside it - because the Muslim is the brother of the Muslim and he should guide him if he makes a mistake, teach him if he is ignorant and correct him if the Qur'an is difficult for him.
Source:
Ibn Baz
Fatawa Islamiyah, Vol. 2 Pages 410-411