(And they will say) i.e. the disbelievers of Mecca: (If only a portent) a sign (were sent down upon him) i.e. to Muhammad (pbuh) (from his Lord!) to vouchsafe his claim. (Then say) O Muhammad: (The Unseen) regarding the sending of such a sign (belongeth to Allah. So wait) for my destruction! (Lo, I am waiting with you) for your destruction.