The group known as the Islamic State (in Iraq and al-Sham) or ISIS has gained notoriety for its cruelty and violence. ISIS has been condemned by most mainstream Muslim scholars, both for their premature declaration of an Islamic caliphate, as well as their many contraventions of Islamic law.
A recent open letter signed by 126 leading Muslim scholars to the leader of ISIS, known as Abu Bakr al-Baghdadi, sets out twenty-four points on which the group has contravened Islamic law, including killing innocent civilians, mistreating People of the Book, reintroducing slavery, cruelly killing captives and mutilating their bodies, involving children in war, and using torture. The letter is signed by Muslim scholars from around the world, including Sheikh Abdullah b. Bayyah, Sheikh Muhammad al-Ya'qubi, as well as leading scholars from Iraq, Syria, Egypt, Yemen, Lebanon, Palestine, Indonesia, India, Pakistan, Malaysia, Morocco, Mauritania, and Sudan.
The letter is available at in English at http://www.lettertobaghdadi.com/14/english-v14.pdf and Arabic at http://www.lettertobaghdadi.com/14/arabic-v14.pdf.
Sheikh Abdullah b. Bayyah, Vice President of the International Union of Muslim Scholars (IUMS or Ittihad al-Alami li-ʿUlama' al-Muslimin), has also released a statement entitled "This is Not the Path to Paradise: Response to ISIS". In it he condemns the destruction of holy places, fighting for reasons other than self-defense and repelling aggression, and the excommunication of Muslims (takfir). Sheikh Bin Bayyah also clarifies that the caliphate is not a matter of theology and that for many centuries many Muslim lands were independent of the caliphate.
Other groups who have condemned ISIS include the opposition Syrian Islamic Council (al-Majlis al-Islami al-Suri), who released a statement detailing its many contraventions of Islamic law, its oppression and aggression against person and property, and says that ISIS is a "gang of rebellion, extremism, and excommunication". The statement is translated at this link.
A fatwa by the Syrian Islamic Organisation (Hay'at al-Sham al-Islamiyya) condemns ISIS' practice of killing captives with a knife.
The Historic Islamic Edict (Fatwa) on Joining ISIS/ISIL by the Canadian Muslim Imams and scholars is a Fatwa regarding ISIS/ISIL and its recruitment activities.
An online magazine has been launched by British imams as an attempt to combat extremist groups such as ISIS. The magazine's first edition, called Exposing the Truth About ISIS, attempts to prove the illegitimacy of ISIS's declared caliphate and quotes religious texts to show how extremist groups attempt to mislead Muslim youth.
Shaykh Muhammad al-Yaqoubi, Syrian Scholar condemns ISIS in an interview with CNN's Christiane Amanpour.