Passage 9.26.3
λέγεται δὲ καὶ ὡς νόθη Λαΐου θυγάτηρ εἴη, καὶ ὡς τὸν χρησμὸν τὸν Κάδμῳ δοθέντα ἐκ Δελφῶν διδάξειεν αὐτὴν κατὰ εὔνοιαν ὁ Λάιος· ἐπίστασθαι δὲ πλὴν τοὺς βασιλέας οὐδένα ἄλλον τὸ μάντευμα. ὁπότε οὖν τῇ Σφιγγὶ ἀμφισβητήσων τις ἀφίκοιτο τῆς ἀρχῆς---γενέσθαι γὰρ τῷ Λαΐῳ ἐκ παλλακῶν υἱοὺς καὶ τὰ χρησθέντα ἐκ Δελφῶν ἐς Ἐπικάστην μόνην καὶ τοὺς ἐξ ἐκείνης ἔχειν παῖδας---, τὴν οὖν Σφίγγα χρῆσθαι σοφίσμασιν ἐς τοὺς ἀδελφούς, ὡς τὸν Κάδμῳ γενόμενον χρησμὸν εἰδεῖεν ἂν Λαΐου γε ὄντες·
It is also said that she was an illegitimate daughter of Laius, and that Laius himself, out of affection, taught her the oracle delivered to Cadmus at Delphi. Moreover, no one else knew this oracle except the royal family. Thus, whenever anyone challenged the right to royal power and approached the Sphinx—for Laius had sons born from concubines, and the oracle from Delphi had declared that rule belonged only to Epicaste and her offspring—the Sphinx accordingly employed subtle riddles against these brothers, reasoning that as sons of Laius, they must surely know the oracle given to Cadmus.