What is the ruling on praying in the second congregational prayer in the mosque if one fears that the imam of the second congregation makes mistakes in ablution or pronunciation in the Faatihah, has very thin or transparent clothes, or makes other common mistakes that render the prayer invalid? Must one enquire or go into details about these things? What if one only finds out after he prayed and finished?

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

It is not permissible to repeat the prayer just out of fear that the imaam made a mistake in ablution or in other matters that you mentioned in the question. It is an obligation upon the Muslim to think good of other Muslims in general, and especially about the imaams, unless he has evidence regarding what he thinks. Allah forbade us from negative assumption; Allah says (what means): {O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin.} [Quran 49:12]

Ibn Katheer interpreted the negative assumption that is forbidden to mean accusing people and being suspicious about them without reason. At-Tabari when interpreting this verse reported, “Ibn ‘Abbaas said, 'Allah forbade a believer to think evil of another believer.' Then he said, 'If a believer thinks evil and does not think good, then he is sinful because Allah forbade him from this.'”

Also, the Prophet said, “Beware of suspicion, for suspicion is the worst of false tales.” [Al-Bukhari and Muslim]

Ibn Abu Shaybah reported in Al-Musannaf, and Al-Bayhaqi in Ash-Shu'aab, and Al-Albaani classified it as authentic in As-Silsilah As-Saheehah that the Prophet looked at the Ka’bah and said, “How sacred you are! How great is your right! However, the Muslim is more sacred than you; Allah has made sacred his [Muslim’s] wealth, honor and his blood, and He has forbidden harming him or thinking evil of him.”

The person led in prayer is not required to investigate if his imaam is upon complete purity or the like; rather, it is not permissible for him to ask about this, as this is accusing the Muslim and thinking evil of him. Rather, he should treat him according to his apparent state. If he knows that his imaam had violated one of the conditions for the validity of the prayer, such as not having performed ablution or not covering his ‘Awrah (what should be covered of the body), and then he prayed behind him, then, in this case, his prayer is invalid and he must repeat it.

As regards other people led in prayer (congregation) and who do not know what this person knows about the imaam, then the scholars differed in opinion about them; the most correct opinion is that the prayer of those who do not know is valid, as they are excused because of ignorance (since they do not know) and because of the legal maxim that if someone does something properly according to Islamic evidence, then his act cannot be invalidated except with Islamic evidence as well.

One must also repeat the prayer if the imaam does not recite the Faatihah properly while the person led in prayer can recite it properly. It is not valid to be led by such an imaam according to the majority of scholars unless the person led in prayer is like the imaam, as it is valid for the person led in prayer to be led by someone like him. The same thing applies if the imaam makes mistakes that change the meaning in the recitation, such as him saying An’amtu ‘Alayhim (upon whom I have bestowed favor) instead of saying An’amta Alayhim (upon whom You have bestowed favor). So it is not valid to be led by someone who makes such mistakes. Or when he changes a letter (from the Faatihah) with another letter, such as changing the letter “r” to “gh” or “l”.

The person who prays behind an imaam who makes such mistakes must repeat the prayer. However, according to the most preponderant opinion of the scholars, it is not a condition for the validity of the prayer that the reciter be a person who is proficient in the recitation of the Quran (Tajweed).

To conclude, a Muslim should not think evil of his fellow Muslim, and he should not take doubts into consideration, as this may lead him to having Waswaas (obsessive whispers) and opening the door to the devil, who would then make him doubt about everything.

He should deal with the imaam and other people according to what appears from them, as ‘Umar Ibn Al-Khattaab, may Allah be pleased with him, said, “Verily, at the time of the Prophet people would be judged by the divine revelation, but the divine revelation has ceased. Now we judge you according to your outward deeds. Whoever shows us good, we will trust him and favor him, and it is not for us to judge his inner secrets, for Allah will hold him accountable for those. Whoever shows us evil, then we will not trust him or believe in him even if he claims that his intention is good.” [Al-Bukhari]

Allah knows best.