The terror group ISIS (which calls itself „islamic State“ now) „proudly“ uploads video clips of it´s fighters beheading captured or killed enemy soldiers. Since this group considers itself Salafi and thus „true Muslims“ (something they claim ALL Shias and many Sunnis are not) one should make a fact-check regarding their actions.

There is no clear reference to the act of beheading in the Quran, however, there is a single verse that protagonists of beheading point to in order to justify this barbaric act:
„If you encounter (in war) those who disbelieve, you may strike the necks. If you take them as captives you may set them free or ransom them, until the war ends. “   47:4

In my opinion „strike the necks“ in battle is not the same as beheading disarmed and subdued prisoners of war in a way that sheep are slaughtered. In times when the major short distance weapon of war were swords striking the neck – as brutal as it appears today – was the most effective way of killing enemies that usually had body armor. At any rate it was not more brutal, painful or inhuman than stabbing into the enemies torso.
Looking at the second sentence of the verse it becomes clear that neither the „believers“ were strictly commanded to kill the enemies nor was any beheading of the prisoners as punishment allowed. The verse explicitly allows for taking captives and even recommends to set them free or ransom them.
ISIS is not fighting against its opponents in close range hand-to-hand combat. Besides, there is no legitimate way to interpret the above verse as approval to behead unarmed and handcuffed prisoners who pose no danger.

Beheading enemies, especially cutting off the heads of dead enemies is an act of mutilation.
What is the position of the Ahadith (prophetic traditions) on this?

(1) Al-Hayyaj ibn Imran reported: A servant boy of Imran had ran away and he vowed by Allah that if he caught him, he would cut off his hand. Then he sent me to ask about that, so I came to Samurah ibn Jundab and I asked him. He said, “The Messenger of Allah, peace and blessings be upon him, encouraged us to give in charity and he forbade us from mutilation.” So I went to Imran ibn Hussein and I asked him. He said, “The Messenger of Allah, peace and blessings be upon him, encouraged us to give in charity and he forbade us from mutilation.”

Source: Sunan Abu Dawud 2667

(2)The people of knowledge hate mutilation.