The story is from a Turkish english news website, Daily Sabah, refering to an investigotary piece in the Financial Times.

It states that ISIS has carried out house to house raids to arrest those foreigners who have not reported for duty and has executed around a hundred.

If you find that you made a mistake by going to Syria and ended up joining the wrong group (or joined the right group but after its defeat to ISIS you were given the choice of joining ISIS or death), you now have the options to either stay and die, try leaving and die or succeed in leaving and end up in prison.