Passage 5.7.9
Ἀριστέας δὲ γὰρ ὁ Προκοννήσιος---μνήμην γὰρ ἐποιήσατο Ὑπερβορέων καὶ οὗτος---τάχα τι καὶ πλέον περὶ αὐτῶν πεπυσμένος ἂν εἴη παρὰ Ἰσσηδόνων, ἐς οὓς ἀφικέσθαι φησὶν ἐν τοῖς ἔπεσιν. Ἡρακλεῖ οὖν πρόσεστι τῷ Ἰδαίῳ δόξα τὸν τότε ἀγῶνα διαθεῖναι πρώτῳ καὶ Ὀλύμπια ὄνομα θέσθαι· διὰ πέμπτου οὖν ἔτους αὐτὸν κατεστήσατο ἄγεσθαι, ὅτι αὐτός τε καὶ οἱ ἀδελφοὶ πέντε ἦσαν ἀριθμόν.
Aristeas of Proconnesus—for he also made mention of the Hyperboreans—likely learned even more about them from the Issedonians, whom he claims in his poem to have visited. Thus, the tradition attributes to Heracles the Idaean the first establishment of the contest at that time, as well as its naming as "Olympic." He ordained that it should be held every fifth year because he himself and his brothers numbered five in all.