﴿ فَلْيَأْتُوا۟ بِحَدِيثٍۢ مِّثْلِهِۦٓ إِن كَانُوا۟ صَٰدِقِينَ ﴾

“But then, [if they deem it the work of a mere mortal,] let them produce another discourse like it - if what they say be true!”

Then let them bring a concocted discourse like it if they are truthful in what they say.