and if ye are in doubt as to what we have revealed from time to time to our servant then produce a sura like thereunto and call your witnesses or helpers besides allah if your are true.
وإن كنتم في ريب مما نزلنا على عبدنا فأتوا بسورة من مثله وادعوا شهداءكم من دون الله إن كنتم صادقين