Below I have translated the document circulated by the Islamic State's al-Eftaa wa al-Buhuth committee on the subject of the burning alive of the Jordanian pilot. This committee is responsible for providing Islamic textual justifications for various decrees on the acceptability of certain acts. This latest example is perhaps the most notorious. It is important that these documents be brought to light because as the corpus of Islamic texts- whether verses of Qur'an, the ahadith and acts from early Islamic history- is so vast, the Islamic State will likely find some reference that can justify its actions and make its supporters and members feel more sure of themselves. My friend Hassan Hassan has already noted this problem, and it presents a significant challenge to those who wish to counter the Islamic State on interpretation and counter-interpretation grounds.
Question: What is the ruling on burning the kafir [disbeliever] with fire until he dies?
Answer: [...] The Hanafis and Shafi'is* have permitted it, considering the saying of the Prophet 'Fire is only to be administered as punishment by God' as an affirmation of humility. Al-Muhallab** said: "This is not an abslolute prohibition, but rather on the path of humility."
Al-Hafiz ibn Hajar*** said: "What points to the permissibility of burning is the deeds of the Companions, and the Prophet put out the eyes of the Uraynians with heated iron...while Khalid ibn al-Waleed**** burnt people of those who apostazied."
And some of the Ahl al-'Ilm have been of the opinion that burning with fire was prohibited originally, but then on retaliation it is permitted, just as the Prophet did to the people of Urayna, when he put out the eyes of the Uraynians with fire- in retaliation- as is related in Sahih [reliable] tradition, and this brought forth the words together among the proofs.
*- Two of the four main schools of Sunni jurisprudence, the others being Maliki and Hanbali.
**- Early theologian in Islam- died c. 702 CE.
***- Fifteenth century imam and jurist from Egypt: bio here.
****- Companion of Muhammad who participated in early Muslim conquests in the Levant.
(5 February 2015): For clarity, 'retaliation' is in the literal sense of 'eye for an eye'. Consider this explanation on the burning of the Jordanian pilot from an IS source in Ninawa province:
"They say that IS burnt Muadh the apostate alive, and have forgotten that Muadh is the one who was burning children of the Muslims alive. [This is when] he was bombing them with heat missiles."