Diogenes Laertius, Lives of the Eminent Philosophers 9.5-6
Diogenes is discussing Heraclitus:
τὸ δὲ φερόμενον αὐτοῦ βιβλίον ἐστὶ μὲν ἀπὸ τοῦ συνέχοντος ‘περὶ φύσεως’, διῄρηται δ’ εἰς τρεῖς λόγους, εἴς τε τὸν περὶ τοῦ παντὸς καὶ πολιτικὸν καὶ θεολογικόν. ἀνέθηκε δ’ αὐτὸ εἰς τὸ τῆς Ἀρτέμιδος ἱερόν, ὡς μέν τινες, ἐπιτηδεύσας ἀσαφέστερον γράψαι, ὅπως οἱ δυνάμενοι <μόνοι> προσίοιεν αὐτῷ καὶ μὴ ἐκ τοῦ δημώδους εὐκαταφρόνητον ᾖ.
The book which is attributed to him is On Nature, a continuous work but divided into three sections: one on the whole cosmos, one on politics and one on theology. He dedicated it in the temple of Artemis; according to some people he deliberately wrote it in a rather obscure way, so that only those who had the ability would approach it, and so that familiarity should not breed contempt.