Plato, Laws 742b

τινος ἀναγκαίας ἄλλης τῇ πόλει κηρυκείας, ἐκπέμπειν τινὰ ἂν δέῃ, τούτων χάριν ἀνάγκη ἑκάστοτε κεκτῆσθαι τῇ πόλει νόμισμα Ἑλληνικόν. ἰδιώτῃ δὲ ἂν ἄρα ποτὲ ἀνάγκη τις γίγνηται ἀποδημεῖν, παρέμενος μὲν τοὺς ἄρχοντας ἀποδημείτω, νόμισμα δὲ ἄν ποθεν ἔχων ξενικὸν οἴκαδε ἀφίκηται περιγενόμενον, τῇ πόλει αὐτὸ καταβαλλέτω πρὸς λόγον ἀπολαμβάνων τὸ ἐπιχώριον· ἰδιούμενος δὲ ἄν τις φαίνηται, δημόσιόν τε γιγνέσθω καὶ ὁ συνειδὼς καὶ μὴ φράζων ἀρᾷ καὶ ὀνείδει μετὰ τοῦ ἀγαγόντος ἔνοχος ἔστω, καὶ ζημίᾳ

If a private citizen ever finds himself obliged to go abroad, he may do so, after first getting leave from the magistrates; and should he come home with any surplus of foreign money, he shall deposit it with the State, and take for it an equivaIent in home coinage; but should anyone be found out keeping it for himself, the money shall be confiscated, and the man who is privy to it and fails to inform, together with the man who has imported it, shall be liable to cursing and reproach and, in addition, to a fine not less than the amount of the foreign money