and if you are in doubt as to that which we have revealed to our servant then produce a chapter like it and call on your witnesses besides allah if you are truthful.
وإن كنتم في ريب مما نزلنا على عبدنا فأتوا بسورة من مثله وادعوا شهداءكم من دون الله إن كنتم صادقين