LSJ
οῦ, ὁ, the Ilissus, in Attica, Pl. Criti. 112a, Hdn. Gr. 1.213, etc. ; Ἱλισός, the god I., IG1². 324.89, cf. 310.206; Ἰλισσός freq. f.l. in codd., as Hdt. 7.189, Str. 9.1.24, A.R. 1.215, D.P. 424; Εἰλισσός f.l. in Paus. 1.19.5.
Liddell-Scott-Jones, Greek-English Lexicon (9th ed., 1940)