The flag of the Syrian jihadi-insurgent group Hurras al-Din and a fighter wears another, better-known flag badge on his chest.
UPDATE (January 17, 2020):
Arm badge of the “Black Flag” made most (in)famous by Islamic State worn by a fighter in the Incite the Believers Operations Room rebel umbrella in Syria. Though the flag is most widely associated with Islamic State it is also used by a number of other Sunni militant Islamist groups including Al-Qa’ida-affiliated groups.
UPDATE (January 19, 2020):