Passage 6.20.14
τὸ μὲν δὴ ἐξ ἀρχῆς Κλεοίτας ἐστὶν ἄφεσιν μηχανησάμενος, καὶ φρονῆσαί γε φαίνεται ἐπὶ τῷ εὑρήματι, ὡς καὶ ἐπίγραμμα ἐπὶ ἀνδριάντι τῷ Ἀθήνῃσιν ἐπιγράψαι ὃς τὴν ἱππάφεσιν ἐν Ὀλυμπίᾳ εὕρατο πρῶτος, τεῦξέ με Κλεοίτας υἱὸς Ἀριστοκλέους· Κλεοίτα δέ φασιν ὕστερον Ἀριστείδην σοφίαν τινὰ καὶ αὐτὸν ἐς τὸ μηχάνημα ἐσενέγκασθαι.
The starting device was originally a contrivance of Cleoitas, who indeed seemed proud of his invention, as he even inscribed an epigram on his statue in Athens stating he was the first to devise the horse-racing starting gate at Olympia: "Cleoitas, son of Aristocles, made me." They say that afterwards Aristides himself also contributed some skill towards the improvement of the apparatus.