Passage 3.7.1
γένους μὲν δὴ τοῦ Εὐρυσθένους, καλουμένων δὲ Ἀγιαδῶν, Κλεομένης ὁ Λεωνίδου βασιλεὺς ὕστατος ἐγένετο ἐν Σπάρτῃ· τὰ δὲ ἐς τὴν οἰκίαν τὴν ἑτέραν τοιάδε ἤκουσα εἶναι. Προκλῆς ὁ Ἀριστοδήμου τῷ παιδὶ ὄνομα τίθεται Σόον. Εὐρυπῶντα δὲ τὸν Σόου φασὶν ἐς τοσοῦτον ἀφικέσθαι δόξης ὡς καὶ τὴν οἰκίαν ταύτην Εὐρυπωντίδας ὄνομα ἀπʼ αὐτοῦ λαβεῖν, Προκλείδας ἐς ἐκεῖνον καλουμένους.
Cleomenes, son of Leonidas, was the last king at Sparta from the line of Eurysthenes, known as the Agiadae. Regarding the other royal house, I have heard the following account. Procles, son of Aristodemus, named his son Soüs. Eurypontes, the son of Soüs, gained such great renown that this royal house took its name from him and was thereafter called the Eurypontidae, although before him it had been known as the Procleidae.