The earliest I could find was Dionysius of Halicarnassus:Shirley Rollinson wrote: ↑October 2nd, 2021, 7:42 pm as a matter of curiosity, when did the term "κοινη" come into use to indicate the language?
ἡ δὲ λέξις, ᾗ κέχρηται, τοιοῦτόν τινα χαρακτῆρα ἔχει. καθαρὰ μέν ἐστιν οὐχ ἧττον τῆς Λυσίου καὶ οὐδὲν εἰκῇ τιθεῖσα ὄνομα τήν τε διάλεκτον ἀκριβοῦσα ἐν τοῖς πάνυ τὴν κοινὴν καὶ συνηθεστάτην... (Isoc. 2.1)
Dionysius of Halicarnassus. (1899). Dionysii Halicarnasei Quae Exstant, Volumen Quintum Opsuculorum, Volumen Prius. (H. Usener, Ed.). Medford, MA: In Aedibus B.G. Teubneri.