His address to monks, rendered from Coptic, exhorting them to perseverance, and encouraging them against the wiles of Satan.
43 'And for your fearlessness against them hold this sure sign— whenever there is any apparition, be not prostrate with fear, but whatsoever it be, first boldly ask, Who are you? And from whence do you come? And if it should be a vision of holy ones they will assure you, and change your fear into joy. But if the vision should be from the devil, immediately it becomes feeble, beholding your firm purpose of mind. For merely to ask, Who are you? And whence do you come? Is a proof of coolness. By thus asking, the son of Nun learned who his helper was; nor did the enemy escape the questioning of Daniel. '
Source: Vita S. Antoni (Life of St. Anthony) (New Advent)