In a congregational prayer, the imam is in front of the people before Allah and is leading them in the most important ritual of Islam, namely the prayer. Hence his prayer is very important and the prayer of those who are praying behind him is connected to it in many rulings, the aims of which are summed up in the hadeeth of Abu Hurayrah (may Allah be pleased with him) who said: The Messenger of Allah (blessings and peace of Allah be upon him) said: The imam is liable and the muezzin is entrusted. O Allah, guide the imams and forgive the muezzins.
Narrated by Abu Dawood, 517; classed as saheeh by al-Albaani in Saheeh Abi Dawood.
The words of the Prophet (blessings and peace of Allah be upon him), The imam is liable include many of the meanings of liability or responsibility on which the scholars are unanimously agreed with regard to prayer in congregation.
The imam is liable means that it is obligatory for him to protect the prayer of those who are praying behind him from becoming invalid and to remember the number of rakahs; he should not peck in the prayer (i.e., move too quickly), which undermines the pillars of the prayer, and he should not fall short in fulfilling the conditions of the prayer; he should make sure that he does the Sunnahs and postures of prayer properly, and so on.
The imam is liable means that he takes on behalf of those who are praying behind him the responsibility of reciting out loud in the prayers where recitation is to be done out loud, and the responsibility of reciting the short soorahs (after al-Faatihah) too. He also covers the mistake of the one who prays behind him if he omits some of the Sunnahs and even if he omits reciting al-Faatihah if he comes late to the prayer. All of that is covered in the idea of liability or responsibility, on which there is scholarly consensus.
The imam is liable also means that he is responsible for offering supplication for all those who pray behind him if he says Qunoot or offers supplication for them. He is responsible for teaching those who prayed behind him the rulings on prayer so that it will not be spoiled and they will not be deprived of the full reward.
Al-Shawkaani (may Allah have mercy on him) said:
The imam is liable -- liability or responsibility includes the meanings of sponsorship, protection and care.
What is meant is that they - i.e., imams - are liable or responsible when they recite Qur'aan and adhkaar quietly. That was narrated from al-Shaafai in al-Umm.
And it was said that what is meant is the responsibility to make sure that his supplication includes all people, not only himself.
And it was said that it is because he takes care of standing and reciting for the one who joins the prayer late. End quote.
Nayl al-Awtaar, 2/42.
Nevertheless the scholars did not understand from this hadeeth what some people understand, which is that if the imam's prayer is valid the prayer of those who are praying behind him is also valid no matter what mistakes they make, and that if the imam's prayer becomes invalid, then the prayer of the those who are praying behind him also becomes invalid even if they did all the essential parts and fulfilled all the conditions.
Rather the scholars say that mistakes that may be made by the one who is praying behind the imam in his prayer are either of two types:
1. Either it is the kind of mistake which invalidates the prayer, such as loosing one's wudoo', eating, drinking, laughing and other things that invalidate the prayer. That also includes omitting any essential part of the prayer. Such mistakes render the prayer of the one who is praying behind the imam invalid and the imam cannot cover any of that, according to scholarly consensus.
2. Or it is the kind of mistake that does not invalidate the prayer, such as omitting some Sunnahs or aspects of postures, or making some mistakes such as turning the head, smiling and the like, which does not make the prayer invalid. Or it may include forgetting some obligatory parts of the prayer, such as forgetting to recite the first tashahhud or the tasbeeh when bowing and prostrating, and the like. These are the mistakes that may be covered by the imam's prayer, and the reward for praying in congregation may cover shortcomings and mistakes that a person may make.
There are some fiqhi issues concerning which the fuqaha' of the four madhhabs differed, according to their differences about the meaning of the words of the Prophet (blessings and peace of Allah be upon him), The imam is liable. These may be seen in the books that discuss differences of scholarly opinion.
And Allah knows best.
Reproduced from Islam QA