Plato, Laws 730a

δὲ διαφερόντως ὁ ξένιος ἑκάστων δαίμων καὶ θεὸς τῷ ξενίῳ συνεπόμενοι Διί. πολλῆς οὖν εὐλαβείας, ᾧ καὶ σμικρὸν προμηθείας ἔνι, μηδὲν ἁμάρτημα περὶ ξένους ἁμαρτόντα ἐν τῷ βίῳ πρὸς τὸ τέλος αὐτοῦ πορευθῆναι. ξενικῶν δʼ αὖ καὶ ἐπιχωρίων ἁμαρτημάτων τὸ περὶ τοὺς ἱκέτας μέγιστον γίγνεται ἁμάρτημα ἑκάστοις· μεθʼ οὗ γὰρ ἱκετεύσας μάρτυρος ὁ ἱκέτης θεοῦ ἔτυχεν ὁμολογιῶν, φύλαξ διαφέρων οὗτος τοῦ παθόντος γίγνεται, ὥστʼ οὐκ ἄν ποτε ἀτιμώρητος πάθοι ὁ τυχὼν ὧν ἔπαθε.

and these follow in the train of Zeus Xenios. Whoso, then, is possessed of but a particle of forethought will take the utmost care to go through life to the very end without committing any offence in respect of Strangers. Of offences against either Strangers or natives, that which touches suppliants is in every case the most grave; for when a suppliant, after invoking a god as witness, is cheated of his compact, that god becomes the special guardian of him who is wronged, so that he will never be wronged without vengeance being taken for his wrongs.