Passage 2.18.2
ὕστερον δὲ οὐκ ἔχω σαφὲς εἰπεῖν πότερον ἀδικίας ἦρξεν Αἴγισθος ἢ προϋπῆρξεν Ἀγαμέμνονι φόνος Ταντάλου τοῦ Θυέστου· συνοικεῖν δέ φασιν αὐτὸν Κλυταιμνήστρᾳ παρθένῳ παρὰ Τυνδάρεω λαβόντα. ἐγὼ δὲ καταγνῶναι μὲν οὐκ ἐθέλω φύσει σφᾶς γενέσθαι κακούς· εἰ δὲ ἐπὶ τοσοῦτον αὐτοῖς τὸ μίασμα τὸ Πέλοπος καὶ ὁ Μυρτίλου προστρόπαιος ἠκολούθησε, τούτοις ἦν ἄρα ὁμολογοῦντα, ἡνίκα ἡ Πυθία Γλαύκῳ τῷ Ἐπικύδους Σπαρτιάτῃ, βουλεύσαντι ἐπίορκα ὀμόσαι, καὶ τοῦδε εἶπεν ἐς τοὺς ἀπογόνους κατιέναι τὴν δίκην.
But afterwards I cannot say clearly whether Aegisthus first committed an act of injustice or whether the murder of Tantalus, the son of Thyestes, preceded this act of Agamemnon. They say that Tantalus had already lived with Clytemnestra while she was still unmarried, taking her from Tyndareus. Yet I do not wish to judge them as by nature wicked; but if to this extent the pollution of Pelops and the vengeful curse of Myrtilus followed upon them, it would confirm what the Pythian priestess said to Glaucus, son of Epicydes, the Spartan, when he sought counsel about swearing a false oath, that the punishment would descend upon his descendants as well.