Hic Rhodus, hic salta
Hic Rhodus, hic salta (Latin) — This is Rhodes, jump that.
Originating in Aesop's Fables a boastful athlete states that he once achieved an incredible long jump in competition on the island of Rhodes. A bystander challenged him to repeat his performance there and now. The lesson to be learned is not to make statements that cannot be proved in short order.