1- Who are innovators? 2- Is performing prayer behind innovators permitted?3- I want all the answer from Qur'an and sahih Hadith

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.

First of all, one needs to know the Ahlus-Sunnah in order to be able to identify the innovators.

Ahlus-Sunnah means those people who adhere to the Quran, Hadeeth and understand both of them as the companions of the Prophet and their followers have understood them. The Prophet said: “The best people are those of my generation, and then those who will come after them (the next generation), and then those who will come after them (i.e. the third generation after them).” Imran [the narrator] said “I do not know whether the Prophet mentioned two or three generations after his generation. The Prophet added, 'There will be some people after you, who will be dishonest and will not be trustworthy, and will give witness (evidences) without being asked to give witness, and will vow but will not fulfill their vows, and fatness will appear among them.” [Al-Bukhari and Muslim]

The Prophet distinguished his generation and the two generations after him with goodness. Allah and His Prophet are pleased with the companions of the Prophet . Allah Says (what means): {And the first forerunners [in the faith] among the Muhajireen and the Ansar and those who followed them with good conduct - Allah is pleased with them and they are pleased with Him, and He has prepared for them gardens beneath which rivers flow, wherein they will abide forever. That is the great attainment.} [Quran 9:100]

The Prophet warned against all kinds of innovations in the Sharee’ah. He said: “The best of speech is the speech of Allah that is the book of Allah. The best of guidance is the guidance of Mohammed. Of all matters the worst are innovations, and everything new is an innovation, and every innovation is a deviation, and every deviation leads to Hellfire.” [An-Nasaa-i]

Innovations will be rejected and returned back to the one who practices them. The Prophet said: “Whoever introduces into this matter of ours (i.e. Islam) something which is not a part of it, is to be rejected.”

The definition of innovation is as Imam Ash-Shaatibi said: “An invented way in Islam is similar to its legitimate acts. The purpose of innovation is to exaggerate in worship of Allah. Sometimes innovations are in belief and sometimes they are in worship”.

They are of two kinds:

Innovations of sayings;

Innovations of deeds.

These innovations in belief may reach the stage where they become Shirk (Associating partners with Allah). Seeking help from anyone other than Allah, making sacrifice to them, calling them for assistance, making vows to them, etc. are pure acts of Shirk. The right to be worshipped in all forms belongs to None except Allah Alone. Whoever offers them to other than Allah, he commits major Shirk and consequently leaves Islam.

To deny, change or add to any of the pillars of Islam, to slander the Prophet his family, or his companions are all acts of innovation, which reach the stage of Kufr.

The examples for the innovations in worship or in the style of worship are mentioned by Imam Ash-Shaatibi who said: “Adhering to certain methods and manners which are not legislated is an innovation, such as making supplications in a gathering in one tone, or celebrating the birthday of the Prophet or the like. Adhering to certain forms of worship at certain times without any evidence for doing so, such as fasting the fifteenth of Sha'baan and making peculiar night prayers in that night is also an innovation.”

From the above details one could easily distinguish between the Sunni (who is from Ahlus-Sunnah) and the innovator.

2. You should search for an Imam who does not practice innovations. If you cannot find an Imam who follows the Sunnah, then explain to this Imam the truth and warn him against practicing what is not approved by the Prophet .

If the innovation practiced by the Imam reaches Kufr, such as seeking help from other than Allah, or calling others for refuge or making sacrifices to others, etc. then you should not pray behind him, and appointing such a person as Imam is not valid. The same ruling applies to the one who curses or abuses the Companions or blasphemes them for misdeeds, especially those denied by the Quran such as claiming that ‘Aa’ishah committed adultery, or who believes that the Quran is incomplete, or the like. Praying behind anyone who believes in the above matters; the prayer behind such person is not valid.

However, the prayer performed behind an innovator whose innovation(s) does not reach Kufr is acceptable.

Allah knows best.