The Histories · 1.148

Now the Panionion is a sacred place on the north side of Mycale, set apart by common agreement of the Ionians for Poseidon of Helike; and this Mycale is a promontory of the mainland running out Westwards towards Samos, where the Ionians gathering together from their cities used to hold a festival which they called the Panionia. (And not only the feasts of the Ionians but also those of all the Hellenes equally are subject to this rule, that their names all end in the same letter, just like the names of the Persians.) These then are the Ionian cities:

Herodotus of Halicarnassus · trans. G. C. Macaulay, 1890
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