The imam’s pronunciation is defective, and some of the members of the congregation are hiding the fact that they have more knowledge of Quran
Question

Our imam substitutes some letters for others, and among us are those who recite well and have memorized a large portion of the Quran. What is the ruling on the person described leading the prayers? What is the ruling on the member of the congregation who has memorized a lot and recites well, given that he has not clearly stated that he recites well and has memorized a great deal?.
Praise be to Allah.In the answer to question no. 50536 we have stated that the scholars differed concerning leading of the prayers by one whose pronunciation is defective (i.e., he substitutes some letters for others). The correct view is that his leading the prayers is valid, but it is better for the prayers to be led by one who recites correctly. 
Shaykh Muhammad al-Saalih al-‘Uthaymeen said: 
If an illiterate person who does not know al-Faatihah leads another illiterate person like him in prayer (illiterate here means one who does not know how to recite al-Faatihah properly), then his prayer is valid because they are equal in their shortcoming. If an illiterate person leads a literate person (i.e., one who can recite al-Faatihah properly) in prayer, then this is not valid. This is our madhhab. 
The reason for that is that the person praying behind the imam is of a higher status than the imam, so how can one who is higher in status be led by one who is lower? 
The second opinion – which is what was narrated from Ahmad – is that it is valid for an illiterate man to lead a literate man in prayer, but we should avoid that, because it involves going against the words of the Messenger (peace and blessings of Allah be upon him): “The people should be led in prayer by the one among them who has the most knowledge of Quran.”  
If he is able to correct it then his prayer is not valid. 
i.e., if the illiterate man is able to correct faults in his recitation that alter the meaning, but he does not do so, then his prayer is not valid. If he is not able to do so then his prayer is valid, but it is not valid for him to lead anyone in prayer except others who are like him. 
But the correct view is that it is valid for him to lead the prayers in this case, because he is excused, as he is unable to correct his recitation of al-Faatihah. Allah says (interpretation of the meaning): 
“So keep your duty to Allah and fear Him as much as you can” [al-Taghaabun 64:16]
“Allah burdens not a person beyond his scope” [al-Baqarah 2:286]
In some remote regions there is no one who can recite al-Faatihah properly, so you may hear them saying ahdina (instead of ihdina = guide us), and they can only recite it in the manner to which they are accustomed, and are unable to correct the pronunciation. Their prayer is valid. But if a person is able to correct it and does not, then his prayer is not valid, if that changes the meaning. Al-Sharh al-Mumti’, 4/248, 249 
A person who cannot recite well should not be chosen to lead the prayer, even if he is hafiz (i.e., has memorized the entire Quran). Rather the one who is chosen to lead the prayer should be one who can recite well and pronounce the letters properly, and also adhere to all the other rulings of prayer. 
Shaykh Muhammad al-Saalih al-‘Uthaymeen (may Allah have mercy on him) said: 
The words “The one who is most entitled to lead the prayer is the one who has most knowledge of Quran and knows the fiqh of prayer” – does “the one who has most knowledge of Quran” mean the one who recites it in the best manner and recites perfectly, pronouncing each letter properly, or does it mean the one who has memorized the most Quran? 
The answer is that what is meant is the one who recites in the best manner, with proper tajweed. This does not mean the tajweed that is known nowadays… It is not essential to recite Quran with ghunnah and maddaad or to have a beautiful voice. The more beautiful the voice, the better, but it is not essential. 
The phrase “and knows the fiqh of prayer” means: he understands the fiqh of prayer so that if some situation arises during the prayer, such as sahw (forgetting something), he can apply the rulings of sharee’ah. 
This has to do with choosing someone to lead the prayers, i.e., if there is a group or congregation and they want to choose one of their number to lead them in prayer. But if the mosque has a regular imam, then he is more entitled in every case, so long as there is no reason why he should not lead them. Al-Sharh al-Mumti’, 4/205, 206. 
Secondly: 
The one who recites well should not hide the fact and let one who does not recite well lead the prayer, because this is going against the words of the Prophet (peace and blessings of Allah be upon him): “The people should be led in prayer by the one among them who has most knowledge of the Book of Allah…” Narrated by Muslim, 673. 
With regard to the words “the people should be led in prayer”, al-Teebi said: This is a command, i.e., let them be led in prayer… 
Al-Haafiz said in al-Fath: 
It is obvious that choosing the one who has most knowledge of Quran to lead the prayer applies only if he also knows what must be known with regard to prayer. If he is ignorant of that then he should not be appointed to lead the prayer, according to scholarly consensus.