Passage 3.4.1
Δωριεὺς μὲν δὴ---οὐ γὰρ ἠνείχετο ὑπακούειν Κλεομένει μένων ἐν Λακεδαίμονι---ἐς ἀποικίαν στέλλεται· Κλεομένης δὲ ὡς ἐβασίλευσεν, αὐτίκα ἐσέβαλεν ἐς τὴν Ἀργολίδα, Λακεδαιμονίων τε αὐτῶν ἀθροίσας καὶ τῶν συμμάχων στρατιάν. ὡς δὲ ἐπεξῆλθον οἱ Ἀργεῖοι σὺν ὅπλοις, ὁ Κλεομένης ἐνίκα τῇ μάχῃ· καὶ ---ἦν γὰρ πλησίον ἄλσος ἱερὸν Ἄργου τοῦ Νιόβης--- καταφεύγουσιν ὡς ἐτράποντο ὅσον τε πεντακισχίλιοι τῶν Ἀργείων ἐς τὸ ἄλσος. Κλεομένης δὲ---ἐξώρμει γὰρ τὰ πολλὰ ἐκ τοῦ νοῦ---κελεύει καὶ τότε ἐνεῖναι πῦρ τοῖς εἵλωσιν ἐς τὸ ἄλσος, καὶ τό τε ἄλσος ἡ φλὸξ ἐπέλαβεν ἅπαν καὶ ὁμοῦ τῷ ἄλσει καιομένῳ συγκατεκαύθησαν αὖθις οἱ ἱκέται.
Dorieus—since he could not endure to be subject to Cleomenes by remaining in Lacedaemon—departed to establish a colony. Cleomenes, however, as soon as he came to the throne, immediately invaded Argolis, gathering an army from both the Spartans themselves and their allies. When the Argives came forth armed against him, Cleomenes defeated them in battle; nearby was a grove sacred to Argus, son of Niobe, and about five thousand of the Argives fled into this grove after their defeat. Then Cleomenes, who frequently acted from rash impulse, ordered the Helots at that time also to set fire to the grove; thus, the flames overtook the entire grove, and along with its burning the suppliants within it all perished together.