LSJ
loc. of Ἰσθμός ΙΙ. 2, Ar. Fr. 14D. ; — usu.
on the Isthmus; at the Isthmian games, IG1². 77, Pi. O. 13.98, Lys. 19.63, Timocr. 1.10; also ἐν Ἰσθμοῖ Simon. 125 cod. [Ἱσθ- IG1². 77, 829; Ἰθμοῖ SIG 36 A (Delph., iv BC), but Ἰσθμοῖ ib. B (Olymp., v BC); cf. Ἰσθμός.]
Liddell-Scott-Jones, Greek-English Lexicon (9th ed., 1940)