Shaykh Saalih bin Sa’d al-Suhaymee حفظه الله said about ISIS:
“Evil (sins and disobedience of Allāh, etc.) has appeared on land and sea because of what the hands of men have earned (by oppression and evil deeds, etc.), that Allāh may make them taste a part of that which they have done, in order that they may return (by repenting to Allaah, and begging His Pardon).” ● [al-Room 30:41]
Corruption spreads when there is negligence and exaggeration.
Negligence is when the people of deviation and and secularists leave the practicing of their religions. They are callers to evil and corruption .
Exaggeration is similar to the way of the Khawaarij (a sect in Islaam that believes in the obligation of killing other Muslims) and those like them.
Today, some of the foolish in ‘Iraq have announced that they have established an Islaamic caliphate. This has been their nature for eight years now, we know about them in detail!
They have given a hidden allegiance to a caliph in supporting their false and corrupt methodology. All of them have given a hidden allegiance to an unknown caliph!
As a result, today they have announced that they have established a caliphate and have permitted by it, the killing of those who differ from them. This is similar to the beliefs of the Raafidah (Shee’ah) who claim such things for the Mahdee.
So they claim that the newly established caliphate is for all Muslims from around the world, and this has been established after they have killed and slaughtered many lives from the Muslims brothers that are present there in ‘Irāq. This doesn’t mean that we support the Raafidah there in ‘Iraq, No! We say both parties are both misguided and intend no good for the Muslim Ummah.
So it is upon us to warn against both of these misguided parties there in Iraq. You have the Raafidah opposition, all of them have the intent of spreading evil. All of them are misguided, and all of them are at war with Ahlus-Sunnah wal-Jamaa’ah (mainstream Islaam).
So be warned and do not give ISIS your allegiance! The caliph who claims to be in charge is a Dajjaal (an impostor), a liar, and a caller to evilness and falsehood.
They have slaughtered Muslims in Syria and not they have taken their slaughtering to ‘Iraq. They are oppressors who are supported by the oppressors in Syria because they want the oil from them.
So I ask Allaah, the Most High and Exalted, that He Makes His Religion uppermost and that He Aids the Religion and debases the people of evil and innovation and desires, and anyone else who opposes this religion.
They are impostors and be warned from joining them!
● [Summarised answer from Sharh Usool as-Sittah of Imaam Ahmad, 3/9/1435 – 1/6/-2014, Masjid Nabawee in al-Madeenah an-Nabawiyyah]