Herodotus 7.136.1 (tr. Robin Waterfield):
From his house they made their way
up to Susa. The first thing that happened, once they gained an audience with the
king, was that Xerxes' guards ordered them, and tried to force them, to fall down
and prostrate themselves before the king. Their response to this was to declare
that even if the guards were to hurl them headlong down on to the ground they
would never do any such thing, not only because it was not the Greek way to
prostrate oneself before another human being, but also because that was not what
they had come for.
ἐνθεῦτεν δὲ ὡς ἀνέβησαν ἐς Σοῦσα καὶ βασιλέι ἐς ὄψιν ἦλθον, πρῶτα μὲν τῶν δορυφόρων κελευόντων καὶ ἀνάγκην σφι προσφερόντων προσκυνέειν βασιλέα προσπίπτοντας, οὐκ ἔφασαν ὠθεόμενοι ὑπ’ αὐτῶν ἐπὶ κεφαλὴν ποιήσειν ταῦτα οὐδαμά· οὔτε γὰρ σφίσι ἐν νόμῳ εἶναι ἄνθρωπον προσκυνέειν οὔτε κατὰ ταῦτα ἥκειν.