The Peloponnesian War (Spartans near Lykaion)

Author Thucydides
Subject History of Mt. Lykaion and Area

Thucydides, The Peloponnesian War, 5.54.1-2 (5.54.1, 5.54.2) from The Perseus Digital Library.

About the same time the Lacedaemonians marched out with all their people to Leuctra upon their frontier, opposite to Mount Lycaeum, under the command of Agis, son of Archidamus, without any one knowing their destination, not even the cities that sent the contingents.
The sacrifices, however, for crossing the frontier not proving propitious, the Lacedaemonians returned home themselves, and sent word to the allies to be ready to march after the month ensuing, which happened to be the month of Carneus, a holy time for the Dorians.

Greek Text

ἐξεστράτευσαν δὲ καὶ οἱ Λακεδαιμόνιοι κατὰ τοὺς αὐτοὺς χρόνους πανδημεὶ ἐς Λεῦκτρα τῆς ἑαυτῶν μεθορίας πρὸς τὸ Λύκαιον, Ἄγιδος τοῦ Ἀρχιδάμου βασιλέως ἡγουμένου: ᾔδει δὲ οὐδεὶς ὅποι στρατεύουσιν, οὐδὲ αἱ πόλεις ἐξ ὧν ἐπέμφθησαν.
ὡς δ᾽ αὐτοῖς τὰ διαβατήρια θυομένοις οὐ προυχώρει, αὐτοί τε ἀπῆλθον ἐπ᾽ οἴκου καὶ τοῖς ξυμμάχοις περιήγγειλαν μετὰ τὸν μέλλοντα (Καρνεῖος δ᾽ ἦν μήν, ἱερομηνία Δωριεῦσι) παρασκευάζεσθαι ὡς στρατευσομένους.