Passage 1.28.4
καταβᾶσι δὲ οὐκ ἐς τὴν κάτω πόλιν ἀλλʼ ὅσον ὑπὸ τὰ προπύλαια πηγή τε ὕδατός ἐστι καὶ πλησίον Ἀπόλλωνος ἱερὸν ἐν σπηλαίῳ· Κρεούσῃ δὲ θυγατρὶ Ἐρεχθέως Ἀπόλλωνα ἐνταῦθα συγγενέσθαι νομίζουσι. ὡς πεμφθείη Φιλιππίδης ἐς Λακεδαίμονα ἄγγελος ἀποβεβηκότων Μήδων ἐς τὴν γῆν, ἐπανήκων δὲ Λακεδαιμονίους ὑπερβαλέσθαι φαίη τὴν ἔξοδον, εἶναι γὰρ δὴ νόμον αὐτοῖς μὴ πρότερον μαχουμένους ἐξιέναι πρὶν ἢ πλήρη τὸν κύκλον τῆς σελήνης γενέσθαι· τὸν δὲ Πᾶνα ὁ Φιλιππίδης ἔλεγε περὶ τὸ ὄρος ἐντυχόντα οἱ τὸ Παρθένιον φάναι τε ὡς εὔνους Ἀθηναίοις εἴη καὶ ὅτι ἐς Μαραθῶνα ἥξει συμμαχήσων. οὗτος μὲν οὖν ὁ θεὸς ἐπὶ ταύτῃ τῇ ἀγγελίᾳ τετίμηται·
When you descend, not to the lower city, but just beneath the Propylaia, there is a spring of water and, close by, a sanctuary to Apollo in a cave. It is believed that here Apollo lay with Kreousa, the daughter of Erechtheus. They say that Philippides was sent as a messenger to Sparta when the Persians had landed on the Greek mainland. Upon his return, he reported that the Lacedaemonians had delayed their departure, as it was their custom not to march out for battle until the moon was at its full. Philippides also related how Pan appeared to him on Mount Parthenion, declaring his goodwill toward the Athenians and promising to come and aid them in battle at Marathon. Thus, due to this announcement, the god Pan has been honored since that time.