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A qarin (Arabic: قرين qarīn), according to Islamic literature, is a Jinn. Qarins are unique to each individual. Qarin literally means 'constant companion'. A qareen pushes a person to do evil things and to disobey Allah, with the exception of Muhammad.
From a purely Qur'anic point of view, qarins have been mentioned in the following:
- And whosoever turns away from remembering and mentioning the Most Beneficent, we appoint for him Shaytan to be a qarin to him. Sūrat az-Zukhruf (43:36)
Conflicting opinions exist on the faith of the Qarin. One opinion is that the qarin, being a jinni-type creature, if not guided by his human companion on the right path, becomes a shaytan by the command of God. The companion will be what its guide is or more than what the guide is. However the second opinion is that the Qareen is evil and having one’s Qareen become a Muslim was something that was unique to Muhammad.
With regard to the hadith of Muhammad, “There is none of you who does not have a companion (qareen) appointed for him from among the jinn.” They said, “Even you?” He said, “Even me, but Allaah helped me against him fa aslama [or fa aslamu], so he only tells me to do that which is good.”
There are two well-known views that have been reported. Those who read the phrase as fa aslamu said that it means, “So I am safe [aslamu] from his evil and temptation.” Those who read it as fa aslama said that it means, “The qareen became Muslim [aslama] and became a believer, so he only tells me to do that which is good.” 
They differed as to which view is correct. Al-Khattaabi said: The correct version is fa aslamu [so I am safe]. Al-Qaadi Iyaad thought that fa aslama [so he became Muslim] was correct, and this is the preferred version, because he then said, “so he only tells me to do that which is good.” (Reported by Muslim, 2814). And they differed concerning the report that says fa aslama. It was said that it means he submitted in the sense of surrendering, and it appears in this form (fa astaslama – so he surrendered) in reports narrated elsewhere than in Saheeh Muslim. And it was said that it means that he become a Muslim and a believer. This is the apparent meaning.
Abu Na’eem al-Asbahaani said in Dalaa’il al-Nubuwwah (1/185): It was said aslama meaning he believed. Muhammad was the only one whose qareen became a Muslim and a believer.
Based on this, having one’s qareen become a Muslim was something that was unique to Muhammad.
Furthermore, there is no evidence that the Companions (Sahaba) ever tried to convert their Qareen to become a Muslim.
The idea of a qarin appears in pre-Islamic literature as well. One of the seven mu'allaqat—Arabic poems recognized as masterpieces during the pre-Islamic period—uses the word as a metaphor. To describe his tribe's excellence in battle, poet Amr bin Kulthum says that "every tribe has taken fear of us as a qarin [or "constant companion"]," meaning that their fear of Amr's tribe is always present. This goes further to show the origin of the word Qareen, as described in the Arabic dictionary as a 'Companion'.
- IslamQA: "Every person has a constant companion from among the jinn"
- Majmū‘ al-Fatāwá. 1. pp. 359–361.
- http://www.islamqa.com/en/ref/23415 Fatwa by Sheikh Muhammad Al Munajjid
- Islam Question and Answer - Praying for Jinn Companion to Become Muslim