Translation:

23. And if ye are in doubt as to what We have revealed from time to time to Our servant then produce a Surah like thereunto; and call your witnesses or helpers (if there are any) besides Allah if your (doubts) are true.

Notes (Tafseer)

42. How do we know that there is revelation, and that is from God? Here is a concrete test. The Teacher of God's Truth has placed before you many Suras. Can you produce one like it? If there is any one besides God, who can inspire spiritual truth in such noble language, produce your evidence. Or is it that your doubts are merely argumentative, refractory, against your own inner light, or conscience? All true revelation is itself a miracle, and stands on its own merits.