7.1. The qarin (jinn-companion)
Every human being, without exception, has a companion from among the jinn appointed to accompany him; he instructs him to do what is wrong and forbids him to do what is right. Allah, may He be glorified and exalted, tells us of what some of the people of Paradise will say (interpretation of the meaning), "One of them will say: I had a companion on earth,
who used to say: Are you one of those who believe." (al-Saffat 37:51-52) It was narrated that ‘Abdullah ibn Mas‘ud (may Allah be pleased with him) said, "The Messenger of Allah (blessings and peace of Allah be upon him) said: “There is no one among you but Allah has appointed a companion for him from among the jinn.” They said: Even you, O Messenger of Allah? He said: “Even me, but Allah helped me with him and he became Muslim fa aslama, so he only tells me to do good.” [295] Narrated by Muslim, 2814.

‘Iyad said, “This hadith was narrated in two versions. According to one version, the phrase is fa aslamu, which means: ‘so I am safe from him.’ This is the version which was classed as sahih by al-Khattabi and he regarded it as more likely to be correct. Those who read the phrase as fa aslama interpret it as referring to the qarin (jinn-companion), who became Muslim (aslama). In my view, this is more likely to be correct, based on the fact that the Prophet (blessings and peace of Allah be upon him) then said, ‘so he only tells me to do good.” … You should understand that the ummah is unanimously agreed that the Prophet (blessings and peace of Allah be upon him) was protected from the Shaytan, so the Shaytan could not cause any kind of harm to his physical body or cause any kind of waswas (intrusive thoughts) to dominate his mind, or cause him to say anything that he did not want to say. We have discussed this matter more fully in Kitab al-Shifa.” [296] See: Ikmal al-Mu‘allim bi Fawa’id Muslim, 8/350.

Al-Nawawi said, “This hadith tells us that we should be aware of the confusion of the qarin and his whispers and attempts to mislead us. It tells us that we should beware of him as much as possible.” [297] See: Sharh Muslim, 17/158.

7.2. Jinn possession and its impact on a person’s body and mind
The devils try to harm people in many different ways, one of which is possession of his body and entering it. The Book of Allah, may He be glorified and exalted, the Sunnah of His Messenger (blessings and peace of Allah be upon him) and the consensus of the ummah all indicate that a jinni can possess a human and cause him to suffer epilepsy. Allah, may He be glorified and exalted, says (interpretation of the meaning), "Those who consume interest cannot stand on the Day of Resurrection except as one stands who is being beaten by Satan into insanity." (al-Baqarah 2:275) Ibn Jarir said, “That refers to the Shaytan beating him in this world, which is when he beats him and causes him to develop epilepsy as a result of possession.” [298] See: Tafsir Ibn Jarir, 5/38.

Al-Qurtubi said, “This verse proves that those who deny that epilepsy may be caused by the jinn and claimed that it is solely a physical illness, and that the Shaytan cannot physically go through a human or possess him are wrong.” [299] See: Tafsir Ibn Kathir, 3/355.

Ibn Taymiyah said, “The existence of the jinn is proven by the Book of Allah, the Sunnah of His Messenger and the consensus of the early generations and leading scholars of the ummah. Similarly, the fact that a jinni may enter the body of a human is proven according to the consensus of the leading scholars of Ahl al-Sunnah wa’l-Jama‘ah. Allah, may He be glorified and exalted, says (interpretation of the meaning), "Those who consume interest cannot stand on the Day of Resurrection except as one stands who is being beaten by Satan into insanity." (al-Baqarah 2:275)
In al-Sahih it is narrated that the Prophet (blessings and peace of Allah be upon him) said, “The Shaytan flows through the son of Adam like blood." [300] Narrated by al-Bukhari, 7171; Muslim, 2175.

‘Abdullah ibn al-Imam Ahmad ibn Hanbal said, “I said to my father: Some people say that the jinni cannot enter the body of the one who is affected by epilepsy. He said: O my son, they are lying; here he is speaking on the lips of the one who is possessed.” [301] See: Majmu‘ al-Fatawa, 24/276.