Passage 1.17.4
ἐς δὲ τὴν τελευτὴν τὴν Θησέως πολλὰ ἤδη καὶ οὐχ ὁμολογοῦντα εἴρηται· δεδέσθαι τε γὰρ αὐτὸν λέγουσιν ἐς τόδε ἕως ὑφʼ Ἡρακλέους ἀναχθείη, πιθανώτατα δὲ ὧν ἤκουσα· Θησεὺς ἐς Θεσπρωτοὺς ἐμβαλών, τοῦ βασιλέως τῶν Θεσπρωτῶν γυναῖκα ἁρπάσων, τὸ πολὺ τῆς στρατιᾶς οὕτως ἀπόλλυσι, καὶ αὐτός τε καὶ Πειρίθους--- Πειρίθους γὰρ καὶ τὸν γάμον σπεύδων ἐστράτευεν--- ἥλωσαν, καὶ σφᾶς ὁ Θεσπρωτὸς δήσας εἶχεν ἐν Κιχύρῳ.
Regarding the death of Theseus, many different and contradictory accounts have already been told. For some say he remained imprisoned until rescued by Heracles. But the most credible version I heard is this: Theseus, invading the land of the Thesprotians to seize the wife of their king, lost the greater part of his army and was himself captured, along with Pirithous—for he had marched into battle also eager to arrange a marriage for Pirithous. The Thesprotian king bound them and held them captive at Cichyrus.