Passage 9.17.4
Ζήθῳ δὲ μνῆμα καὶ Ἀμφίονι ἐν κοινῷ γῆς χῶμά ἐστιν οὐ μέγα. ὑφαιρεῖσθαι δὲ ἐθέλουσιν ἀπʼ αὐτοῦ τῆς γῆς οἱ Τιθορέαν ἐν τῇ Φωκίδι ἔχοντες, ἐθέλουσι δέ, ἐπειδὰν τὸν ἐν τῷ οὐρανῷ ταῦρον ὁ ἥλιος διεξίῃ· τηνικαῦτα γὰρ ἢν ἀπʼ αὐτοῦ λαβόντες γῆν τῷ Ἀντιόπης μνήματι περιάψωσι, Τι θορεεῦσιν οἴσει καρπὸν ἡ χώρα, Θηβαίοις δὲ οὐχ ὁμοίως. καὶ ἐπὶ τούτῳ φρουρὰν οἱ Θηβαῖοι τότε ἔχουσι τοῦ μνήματος.
The tomb of Zethus and Amphion is a common mound of earth, not large in size. Those who inhabit Tithorea in Phocis wish to carry away earth from this mound, and they do so whenever the sun passes through the constellation Taurus. For at that time, if they take earth from here and place it upon the tomb of Antiope, their land in Tithorea will yield good crops, while the land of the Thebans will not fare likewise. Because of this, the Thebans at that season appoint guards for the tomb.