Passage 9.39.8
ἐνταῦθα δὴ χρὴ πιεῖν αὐτὸν Λήθης τε ὕδωρ καλούμενον, ἵνα λήθη γένηταί οἱ πάντων ἃ τέως ἐφρόντιζε, καὶ ἐπὶ τῷδε ἄλλο αὖθις ὕδωρ πίνειν Μνημοσύνης· ἀπὸ τούτου τε μνημονεύει τὰ ὀφθέντα οἱ καταβάντι. θεασάμενος δὲ ἄγαλμα ὃ ποιῆσαι Δαίδαλόν φασιν---ὑπὸ δὲ τῶν ἱερέων οὐκ ἐπιδείκνυται πλὴν ὅσοι παρὰ τὸν Τροφώνιον μέλλουσιν ἔρχεσθαι--- τοῦτο τὸ ἄγαλμα ἰδὼν καὶ θεραπεύσας τε καὶ εὐξάμενος ἔρχεται πρὸς τὸ μαντεῖον, χιτῶνα ἐνδεδυκὼς λινοῦν καὶ ταινίαις τὸν χιτῶνα ἐπιζωσθεὶς καὶ ὑποδησάμενος ἐπιχωρίας κρηπῖδας.
Here he must drink the water called Lethe ("Forgetfulness"), so that he may forget everything about which he was previously anxious, and afterward he must drink another water, called the water of Mnemosyne ("Memory"), causing him thereby to remember clearly what he has seen below. Having viewed a certain image which is said to have been wrought by Daedalus—an image concealed by the priests from all except those who are preparing to descend to Trophonius—and having seen this image, having made prayers to it and rendered homage, he then proceeds toward the oracle, clothed in a linen tunic girded with ribbons and shod with sandals according to local custom.