ἥδε ἡ ἡμέρα τοῖς Ἕλλησι μεγάλων κακῶν ἄρξει

From LSJ

νᾶφε καὶ μέμνασο ἀπιστεῖν → keep a clear head and remember not to believe a thing (Epicharmus fr. 250)

Source