Passage 3.20.1
ἐν Θεράπνῃ δὲ κρήνην τὴν Μεσσηίδα ἰδὼν οἶδα. Λακεδαιμονίων δὲ ἑτέροις ἐστὶν εἰρημένον τὴν Πολυδεύκειαν ὀνομαζομένην ἐφʼ ἡμῶν, οὐ τὴν ἐν Θεράπνῃ Μεσσηίδα καλεῖσθαι τὸ ἀρχαῖον· ἡ δὲ Πολυδεύκειά ἐστιν αὐτή τε ἡ κρήνη καὶ Πολυδεύκους ἱερὸν ἐν δεξιᾷ τῆς ἐς Θεράπνην ὁδοῦ.
In Therapne I myself saw the spring called Messeïs. According to another tradition of the Lacedaemonians, the spring that in my time is named Polydeucea was originally called Messeïs, not the one in Therapne. Polydeucea itself consists both of the spring and of a sanctuary of Polydeuces, situated on the right side of the road leading to Therapne.