Passage 9.27.6
καὶ Ἡρακλέους Θεσπιεῦσίν ἐστιν ἱερόν· ἱερᾶται δὲ αὐτοῦ παρθένος, ἔστʼ ἂν ἐπιλάβῃ τὸ χρεὼν αὐτήν. αἴτιον δὲ τούτου φασὶν εἶναι τοιόνδε, Ἡρακλέα ταῖς θυγατράσι πεντήκοντα οὔσαις ταῖς Θεστίου συγγενέσθαι πάσαις πλὴν μιᾶς ἐν τῇ αὐτῇ νυκτί· ταύτην δὲ οὐκ ἐθελῆσαί οἱ τὴν μίαν μιχθῆναι· τὸν δὲ ὑβρισθῆναι νομίζοντα δικάσαι μένειν παρθένον πάντα αὐτὴν τὸν βίον ἱερωμένην αὐτῷ.
The Thespians also have a sanctuary of Heracles. His priestess must remain a virgin until the time fixed by custom has passed. The reason given for this is as follows: it is said that Heracles lay with fifty daughters of Thestius, all in the same night, except for one. This single daughter refused to unite herself with him. Considering himself insulted, Heracles decided that she must remain a virgin for the rest of her life, serving him as his priestess.