Explaining the truth of the possibility of Jinn entering the human body and refuting the claims of those denying this 
Source : Fatwas of Ibn Baaz rahimahullaah – alifta.net
The Shaykh of Islam Ibn Taymiyyah (may Allah be merciful to him) also said in (vol.24, pp. 276-277) the following: The existence of Jinn is affirmed in the Book of Allah, the Sunnah of His Messenger (peace be upon him), and according to the consensus of the Salaf (Righteous Predecessors) of this Ummah and its leading scholars. Similarly the fact that the Jinn can enter human bodies is confirmed by the consensus of Ahl-ul-Sunnah wal-Jama`ah. Allah (the Exalted states) says: “Those who eat Ribâ (usury) will not stand (on the Day of Resurrection) except like the standing of a person beaten by Shaitân (Satan) leading him to insanity.”
The Prophet (peace be upon him) said in the Sahih (authentic) Hadith related by Al-Bukhari and Muslim on the authority of Safiyyah (may Allah be pleased with her) that the Prophet (peace be upon him) said: “Satan circulates in the human body as blood does.”
`Abdullah the son of Al-Imam Ahmad ibn Hanbal said:
“I told my father that some people claim that Jinn can never enter the body of a person. He said, ‘O my son, they tell lies. Satan can even speak on the tongue of those whom he possesses.'”
The Shaykh of Islam Ibn Taymiyyah said:
“What he [Imam Ahmad] said is self-evident. The Jinn may possess someone and cause them to speak a language he does not even know. A possessed person may be violently beaten, in a way that even a camel may not endure, yet he neither feels the beating nor is aware of the words he says.” Furthermore, the possessed person may drag a non-possessed person and or the mat he is sitting on, moves things from a place to another, and other practices that confirmed to those who witnessed such behavior that the one who spoke or moved was not a human being.
Then he said: “None of the leading Muslim scholars denied the ability of Jinn to enter the body of a human being. Whoever denies this fact and assumes that Shari`ah denies it has fabricated lies against Shari`ah, as there is no Shari`ah-based evidence that denies this fact.”
(End of quote)
Imam Ibn Al-Qayyim (may Allah be merciful to him) mentioned the following in his book, Zad Al-Ma`ad fi Hadiy Khayr Al-`Ibad, vol. 4, pp. 66-69:
“There are two types of Jinn’s possession of human beings:
– Possession caused by earthly malicious spirits; and
– the other caused by filthy mixtures.
As for the second type, it is the domain of physicians to find what causes it and how to treat it.
Concerning the possession by spirits, the knowledgeable persons among them acknowledge its existence and do not deny it. They also admit that its treatment should be by confronting the celestial, benevolent and honorable spirits with that wicked and malicious ones to ward off the evil they cause and forestall their acts. This is even stated by Hippocrates in some of his books, as he mentioned some cures for possession. However, he said that his cure is only beneficial in case of possession caused by mixture of filthy blends, but not for that caused by evil spirits.
Whereas, ignorant, worthless and disreputable physicians deny the possession by spirits and do not admit that they affect the body of the possessed person relying on no evidence to support their claims. They are only ignorant, as there is no medical proof to prove the impermissibility of this matter. The Jinn’s possession of human beings is an established fact and reality attests to it. However, the physicians’ denial of possession and ascribing this to the occurrence of some filthy blends is partially, not completely true.
Ignorant and atheist doctors started to attribute the possession of human beings as caused only by filthy blends, but anyone with the least knowledge of such spirits and how they affect the body of the possessed will be astonished at the ignorance of such people and the weakness of their perception of such matters.
In the same article Shaykh Ibn Baaz refutes the one who denies Jinn Possession as below
All praise be to Allah, and peace and blessings be upon the Messenger of Allah, his family, his Companions, and those who follow his guidance.
Some short and long articles were published in local newspapers and others in the month of Sha`ban of this year (1407 A.H.) regarding the declaration of some Jinn (creatures created from fire) who possessed Muslim women in Riyadh. They declared that they had become Muslims before me after they had declared this before brother `Abdullah ibn Mushrif Al-`Umary who lives in Riyadh.
Al-`Umary recited some Adhkar (invocations) on the Jinn-possessed lady and talked to the Jinni, reminding him of Allah (Exalted be He) and His Omnipotence as well as frightening him with the grievous sin of injustice. After the Jinni said that he is a Buddhist disbeliever, the shaykh called him to Islam and to come out of the woman whom he was possessing. Accordingly, the Jinn complied and declared his conversion to Islam before Shaykh Al-`Umary. Then, Shaykh Al-`Umary and the guardians of the woman wanted to come to me with the woman to hear the Jinni’s conversion to Islam. When they were all present, I asked the Jinni about the reason for embracing Islam. He spoke on the tongue of the woman but in a manly voice. All this happened in the presence of the woman who was sitting in the chair next to me beside her brother, sister, and Shaykh Al-`Umary as well as some shaykhs who witnessed and heard the speech of the Jinni while openly declaring his conversion to Islam. He said he was a Buddhist from India. On my part, I advised him to observe Taqwa (fear or wariness of offending Allah) and to leave the body of this woman as he is making her suffer. The Jinni submitted to my order and said that he embraced Islam willingly. Therefore, I advised himto call his people to Islam after Allah (Exalted be He) had guided him to the right path. He came out of the woman’s body and his last word was “As-salamu `alaykum” (peace be upon you). Then the woman spoke with her normal voice and felt comfortable and relaxed from the pain he was causing to her. After the lapse of a month or more, she visited me again with her brothers, maternal uncle, and sister and told me that she was very well, her condition was stable and the Jinni had never returned to her. When I asked her about how she felt when she was possessed by this Jinni, she said that she always had deviant ideas contradicting the Shari`ah (Islamic law), in addition to an inclination to the Buddhist religion, and a desire to read its books. However, after this Jinni was cast out of her, she no longer had such ideas, as she returned to her normal state away from such abominable ideas.
I was told that Shaykh `Aly Al-Tantawy denied the occurrence of such matters, claiming that this is a kind of charlatanism and mere lies. He said that the speech that was mentioned on the tongue of the woman might have been recorded, not real. I asked for the tape in which the Jinni had declared that, and was confirmed that such opinions are really his. However, I could not imagine how he could think that what the woman said was recorded even though I, myself, asked the Jinni some questions and he replied. Then how could a person having the least degree of wisdom imagine that a recorded tape can be asked and give answers. This is one of the ugliest abominations and a way of permitting falsehood.