and if you be in doubt whether the book we have sent down to our servant is from us or not then produce at least one surah like this. you may call all your associates to assist you and avail yourselves of the help of any one other than allah. if you are genuine in your doubt do this.
وإن كنتم في ريب مما نزلنا على عبدنا فأتوا بسورة من مثله وادعوا شهداءكم من دون الله إن كنتم صادقين