Full Idea
This Koran could not have been composed by any but Allah. It is beyond doubt from the Lord of the Creation. If they say: 'It is your own invention', say: 'Compose one chapter like it. Call on your false gods to help you!'
Gist of Idea
The Koran is certainly composed by Allah; no one could compose a chapter like it
Source
Mohammed (The Koran [c.622], Ch.10)
Book Reference
Mohammed: 'The Koran', ed/tr. Dawood,N.J. [Penguin 1968], p.67
A Reaction
I find this unpersuasive, firstly because I couldn't imitate the sonnets of Shakespeare either, and secondly because the authority of a text must be asserted outside the text, not within it. Scribble "this is a ten pound note" on a scrap of paper.