The ruling on one who denies the existence of the jinn, and the ruling on praying behind him
Question

Is it permissible to pray behind a person who does not believe in the existence of the jinn?
Praise be to Allah.Firstly:
Many verses and mutawatir hadiths indicate that Allah, may He be glorified and exalted, created the jinn, and that they are accountable and should follow the commands and prohibitions. Some of them are disbelievers and some of them are believers; they will be brought to account on the Day of Resurrection and they will go either to paradise or to hell, exactly like humans.
Allah, may He be exalted, says (interpretation of the meaning):
{And I did not create the jinn and mankind except to worship Me} [Adh-Dhariyat 51:56]
{O company of jinn and mankind, did there not come to you messengers from among you, relating to you My verses and warning you of the meeting of this Day of yours?” They will say, “We bear witness against ourselves”; and the worldly life had deluded them, and they will bear witness against themselves that they were disbelievers} [Al-An`am 6:130].
The fact that we do not see them does not mean that they do not exist, for Allah has given them the characteristic of invisibility. He, may He be glorified and exalted, says:
{Indeed, he [Iblis] sees you, he and his tribe, from where you do not see them} [Al-A`raf 7:27].
Allah, may He be glorified and exalted, has told us in His holy Book about the origin of the jinn, as He says:
{And the jinn We created before from scorching fire} [Al-Hijr 15:27]
{And He created the jinn from a smokeless flame of fire} [Ar-Rahman 55:15].
According to the hadith narrated from `A’ishah (may Allah be pleased with her), the Prophet (blessings and peace of Allah be upon him) said: “The angels were created from light, the jinn were created from fire, and Adam was created from that which has been described to you.” (Narrated by Muslim, 5314)
Allah, may He be glorified and exalted, revealed an entire surah in His Book which speaks about the jinn.
Hence there is no one among the Muslim groups and sects who deny the existence of the jinn; in fact, even the disbelievers such as the Jews and Christians believe in their existence.
No one denies their existence except some doubters and heretics who base their views on speculation, assumption and imagination in all matters. Hence they do not believe in anything for certain, because all they have is possibilities and speculation. {They follow nothing but conjecture and they do nothing but lie } [Al-An`am 6:116].
Despite that, the leaders of these heretics affirm the existence of the jinn. Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said regarding them:
All their denial of that [i.e., belief in the jinn and so on] is only based on pure ignorance, they reject that which they do not comprehend, even before any explanation thereof reaches them.
However, there are senior philosophers who believed in such things, as well as leading physicians such as Hippocrates and others. They believed in the jinn and described epilepsy as being of two types: one which has a physical cause and one which is caused by the jinn. (End quote from ar-Radd `ala al-Mantiqiyyin, p. 470)
Ibn Taymiyah (may Allah have mercy on him) also said:
No group of Muslims disagreed about the existence of the jinn or the fact that Allah sent Muhammad (blessings and peace of Allah be upon him) to them. The majority of non-Muslim groups also affirm the existence of the jinn.
With regard to the People of the Book, the Jews and Christians, they believe in them just as the Muslims do… This is because the existence of the jinn is mentioned in reports which were narrated from all the prophets, and there are so many such texts that it leaves no room for doubt.
What is known for certain is that the jinn are living, rational beings who act on the basis of free will and are, in fact, subject to commands and prohibitions. They are not mere characteristics and symptoms that exist in human beings or others, as some heretics claim…
Hence some of the Mu`tazilah, such as al-Jaba’i, Abu Bakr and others denied that the jinn could enter the body of the epileptic, but they did not deny the existence of the jinn, because the hadiths that mention jinn possession were not as clear as the hadiths which speak of their existence, even though they were mistaken about that. (End quote from Majmu` al-Fatawa, 18/10).
Ash-Shibli (may Allah have mercy on him) said:
No one denied the existence of the jinn apart from a small group of ignorant philosophers, physicians and the like.
With regard to prominent figures among the philosophers and physicians, what is narrated from them is either that they affirm the existence of the jinn, or nothing is reported from them concerning the matter.
What is known from Hippocrates, is that he said regarding some spring of water: It is beneficial for treating epilepsy; I do not mean the epilepsy that is treated by the priests of the temples [meaning jinn possession, which was treated with spiritual remedies]; rather I mean the epilepsy that is treated by the physicians.
And he said: If we compare our [that is, physicians’] medicine with the medicine of the priests, their medicine is like the medicine of old women compared to our medicine!
Those who deny the existence of the jinn have no reliable proof that is indicative of their nonexistence. All they have is lack of knowledge, because in their profession there is nothing to indicate that the jinn exist. (End quote from Akam al-Marjan, 22).
See also the answers to questions no. 2340 and 20666.
Secondly:
The existence of the jinn is proven physically in real life in such a way that cannot be denied. Stories about them are widespread and are known to all people. Allah, may He be glorified and exalted, has told us about them in His Book, and He has told us about them and about their origin and from what they were created, and how their father Iblis disbelieved in Allah, may He be glorified and exalted, refused to obey Him and was arrogant. Then He has told us about how Iblis tempted the father of humanity, Adam (peace be upon him), and how He sent them both down to earth, and made the devils a trial for people.
And He, may He be glorified, has told us that some of them are believers and some of them are disbelievers, and some of them were subjugated and served Allah’s Prophet Sulayman (peace be upon him).
And He, may He be glorified, has told us that some of them believed in the Prophet (blessings and peace of Allah be upon him) and some of them disbelieved in him.
And He, may He be glorified, has told us that the disbelievers among them are given power over the sons of Adam to tempt them, and they whisper to them in order to bar them from the religion of Allah… and there are many other reports about them in the Quran and Sunnah.
So the one who disbelieves the Quran and rejects what Allah, may He be exalted, has stated in His Book about the creation and existence of the jinn, if he is accountable (that is, an adult of sound mind) and knows what he is saying, has gone beyond the bounds of Islam and has disbelieved because of this belief of his. No prayer offered by someone like him is accepted, and neither is his fasting, and no Jumu`ah or prayer in congregation can be prayed behind him, or any prayer, obligatory or supererogatory, until he comes back to Islam, because denying something that is well known from the teachings of Islam constitutes disbelief.
Shaykh Salih al-Fawzan (may Allah preserve him) said:
Denying the existence of the jinn constitutes disbelief and apostasy from Islam, because it is denying something that is mentioned in many places in the Quran and many texts of the Sunnah that reach the level of being mutawatir. Believing in their existence is part of believing in the unseen, because we cannot see them, and in order to prove their existence we rely on sound religious texts. Allah, may He be exalted, says regarding Iblis and his troops:
{Indeed, he [Iblis] sees you, he and his tribe, from where you do not see them} [Al-A`raf 7:27].
As for denying that they can enter the body of a human being [jinn possession], that does not lead to being deemed to be a disbeliever, but it is wrong and is a rejection of what is proven in the religious texts and by real life experience. But because the issue is so subtle, the one who has a different view cannot be deemed to be a disbeliever. However, he is mistaken, because he did not rely on any evidence to deny that; rather he based it on his rational thinking and ability to understand. But reason is not reliable when it comes to matters of the unseen and it cannot be given precedence over the religious texts, except in the view of misguided people.
Shaykh ‘Abdullah al-Ghunayman (may Allah preserve him) said:
Some people nowadays deny the existence of the jinn, but denying the existence of the jinn constitutes disbelief and puts a person beyond the bounds of Islam, because it is a matter that is mentioned in many texts of the Book of Allah and in reports from the messengers. (End quote from Sharh Fat-h al-Majid)
It is unlikely for a Muslim to be unaware of this, or to believe in something different, because it is something that is very well known and is mentioned in many texts of the Quran and Sunnah.
If we assume that this person or someone else denies such a thing, or is unaware of it, it should be explained to him and proof should be established so as to leave him with no excuse and so that his doubts may be dispelled. Then if he persists in denying the existence of the jinn after clear proof has been presented to him, then he is a disbeliever and an apostate from the religion of Islam.
Shaykh Salih Al ash-Shaykh (may Allah preserve him) said:
Believing in the jinn is obligatory; that includes believing in their existence and what Allah, may He be glorified and exalted, has told us about their characteristics in His Book, and what has been soundly narrated from the Prophet (blessings and peace of Allah be upon him) about them.
So whoever denies the existence of the jinn is a disbeliever, because he has rejected the Quran. Therefore – if such a person is unaware – he should be informed of what it says in the verses of the Quran. Then if he denies the existence of the jinn, even though they are mentioned in the Quran, his denial would lead to rejecting the Quran, and thus he becomes a disbeliever. (End quote from Sharh at-Tahawiyyah, 323)
For the one who finds out that someone denies the existence of the jinn, it is not permissible for him to pray behind him, unless he recounts his misguidance and repents to Allah, may He be glorified and exalted.
For more information, please see the answer to question no. 153492.
And Allah knows best.