Passage 5.24.9
ὁ δὲ ἐν τῷ βουλευτηρίῳ πάντων ὁπόσα ἀγάλματα Διὸς μάλιστα ἐς ἔκπληξιν ἀδίκων ἀνδρῶν πεποίηται· ἐπίκλησις μὲν Ὅρκιός ἐστιν αὐτῷ, ἔχει δὲ ἐν ἑκατέρᾳ κεραυνὸν χειρί. παρὰ τούτῳ καθέστηκε τοῖς ἀθληταῖς καὶ πατράσιν αὐτῶν καὶ ἀδελφοῖς, ἔτι δὲ γυμνασταῖς ἐπὶ κάπρου κατόμνυσθαι τομίων, μηδὲν ἐς τὸν Ὀλυμπίων ἀγῶνα ἔσεσθαι παρʼ αὐτῶν κακούργημα. οἱ δὲ ἄνδρες οἱ ἀθληταὶ καὶ τόδε ἔτι προσκατόμνυνται, δέκα ἐφεξῆς μηνῶν ἀπηκριβῶσθαί σφισι τὰ πάντα ἐς ἄσκησιν.
The statue in the Council-chamber is, among all the images of Zeus, the one particularly fashioned to strike terror into wicked men. It has the epithet "Horkios" (Guardian of Oaths), and holds a thunderbolt in each hand. Beside it there is established a custom for athletes, their fathers and brothers, and also their trainers to swear on the flesh of a boar that they will commit no wrongdoing concerning the Olympic Games. Additionally, the athletes themselves solemnly swear further that for ten successive months they have strictly observed every detail of their training.