Passage 9.3.9
ὑπὸ δὲ τῆς κορυφῆς, ἐφʼ ᾗ τὸν βωμὸν ποιοῦνται, πέντε που μάλιστα καὶ δέκα ὑποκαταβάντι σταδίους νυμφῶν ἐστιν ἄντρον Κιθαιρωνίδων, Σφραγίδιον μὲν ὀνομαζόμενον, μαντεύεσθαι δὲ τὰς νύμφας τὸ ἀρχαῖον αὐτόθι ἔχει λόγος.
Below the peak, upon which they have erected the altar, about fifteen stadia downward, there is a cave of the Cithaeronian Nymphs called Sphragidion. It is said that in ancient times the Nymphs used to deliver oracles there.