Praise be to Allah.
If a person cannot recite al-Faatihah well, it is not appropriate for him to be made an imam. That is because the Prophet (blessings and peace of Allah be upon him) said: “The people should be led by the one who has most knowledge of the Book of Allah.” Narrated by Muslim (673).
What is meant by the one who has the most knowledge is the one who is best in recitation and has memorised the most.
With regard to the validity of prayer offered behind one who cannot recite well, if he omits a letter, such as failing to emphasise the doubled consonant in the word “iyyaaka”, or he replaces one letter with another, such as turning the dhal into zaa’, or he makes a mistake that changes the meaning, such as saying “iyyaaki na‘budu” [“You (Alone) we worship” – this error changes the second person masculine into feminine], when he is able to correct this mistake, but he is careless, then his prayer is not valid and prayer offered behind him is not valid either.
This has been explained previously in the answer to question no. 50536
If he is not able to correct the mistake (and recite properly), then the scholars differed concerning the validity of prayers offered behind him. The correct scholarly view is that such prayers are valid, in sha Allah, but it is better for someone else to lead the prayer.
Ibn Hazm (may Allah have mercy on him) said:
With regard to those who have a speech defect or a stammer, those whose mother tongue is not Arabic, and those who make grammatical mistakes, the prayer of those who are led in prayer by them is acceptable, because Allah, may He be exalted, says (interpretation of the meaning): “On no soul does Allah place a burden greater than it can bear” [al-Baqarah 2:286]. So they are not expected to do more than they are able to do. They have done their prayer as they were commanded, and whoever does his prayer as he has been commanded has done well. Allah says (interpretation of the meaning): “No ground (of complaint) can there be against the Muhsinoon (good-doers)” [at-Tawbah 9:91]. What is very strange is those who regard the prayer of one who has a speech defect or who makes grammatical mistakes or who stammers as valid as far as he is concerned, but they regard as invalid the prayer of those who are led by them in prayer and – at the same time – they regard as invalid the prayer of one who prays forgetting that he is in a state of janaabah [major impurity following sexual activity] but regard as valid the prayer of one who is led in him by prayer, even though his prayer is invalid. And Allah is the source of strength. End quote.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked:
I heard someone say that it is not valid for someone who has a speech defect to lead the people in prayer; i.e., prayer offered behind him is not valid, because he has a defect. Is this correct or not?
This is correct according to some scholars. They think that if a person’s speech defect changes the letters, such as changing the letter raa’ to ghayn or laam, and so on, then some of the scholars think that it is not valid for him to lead the prayer, because he is like one who is illiterate, for whom it is not valid to lead anyone in prayer except others who are like him.
Other scholars think that it is valid for him to lead the prayer, because if a person’s prayer is valid, it is valid for him to lead others in prayer. Moreover, he has done what is required of him, which is to strive for the sake of Allah, may He be exalted, as much as he can. Allah, may He be exalted, says (interpretation of the meaning): “So keep your duty to Allah and fear Him as much as you can”[al-Taghaabun 64:16]. If one who is unable to stand can lead in prayer others who are able to stand, then this is a similar case, because each of them is unable to do an essential part of the prayer – the former is unable to stand and the latter is unable to recite properly. This view is the correct one, that leading of the prayer by one who has a speech defect is valid, even if he changes letters, so long as this is his level of ability. Yet despite that, the person who is chosen to lead a group of people in prayer should be one in whom there is no defect (in recitation), so as to be on the safe side and avoid an area concerning which the scholars differed. End quote.
It should be noted that some people have inappropriately strict standards with regard to recitation in prayer, so they may think that the imam has failed to double a consonant, when that is not the case. Rather the most that can be said is that he has not done it properly. If that is the case, it does not make his prayers invalid.
And Allah knows best.