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!

Let them then produce a recital like unto it,- If (it be) they speak the truth!

Then let them produce speech the like thereof, if they are truthful.

Then let them bring a saying like it, if they are truthful.

Then let them produce a statement like it, if they should be truthful.