Passage 10.2.1
ἐπεὶ δὲ ἐς τὴν Εὐρώπην ὁ Περσῶν στρατὸς διέβη, λέγεται τοὺς Φωκέας φρονῆσαι μὲν ὑπὸ ἀνάγκης τὰ βασιλέως, αὐτομολῆσαι δὲ ἐκ τῶν Μήδων καὶ ἐς τὸ Ἑλληνικὸν παρὰ τὸ ἔργον τὸ Πλαταιᾶσι παρατάξασθαι. χρόνῳ δὲ ὕστερον κατέλαβεν αὐτοὺς ζημιωθῆναι χρήμασιν ὑπὸ Ἀμφικτυόνων· οὐδὲ ἔχω τοῦ λόγου τὸ ἀληθὲς ἐξευρεῖν εἴτε ἀδικήσασιν ἐπεβλήθη σφίσιν εἴτε Θεσσαλοὶ κατὰ τὸ ἐκ παλαιοῦ μῖσος γενέσθαι τὴν ζημίαν τοῖς Φωκεῦσιν ἦσαν οἱ πράξαντες.
When the Persian army crossed into Europe, it is said that the Phocians, compelled by necessity, initially sided with the Persian king, but later deserted from the Medes and joined the Greeks just before the battle at Plataea. At a later time, they were penalized by the Amphictyons with a monetary fine. However, I cannot determine with certainty whether this penalty was imposed on them because they genuinely committed wrongdoing, or whether, due to old enmity, it was the Thessalians who arranged for this punishment against the Phocians.