The Ecclesiastical History of Socrates Scholasticus

How the Emperor Theodosius received intelligence of what was done in an incredibly short space of time, and how he was quickly informed of events taking place far away, I shall attempt to explain. For he had the good fortune to possess among his subjects a man endowed with extraordinary energy both of body and mind, named Palladius; who rode so vigorously that he would reach the frontiers of the Roman and Persian dominions in three days, and again return to Constantinople in as many more. The same individual traversed other parts of the world on missions from the emperor with equal celerity: so that an eloquent man once said not unaptly, ‘This man by his speed proves the vast expanse of the Roman Empire to be little.’ The king of the Persians himself was astonished at the expeditious feats which were related to him of this courier: but we must be content with the above details concerning him.