Sindarin Grammar P2: Historical Development

Like all Elvish languages, the origin of Sindarin dates back to the Common Quenderin [CQ] language spoken by the Elves when they awoke beside the shores of Cuiviénen (S. Nen Echui). Even in those early days, the Elves divided themselves into three tribes, and Sindarin is ultimately derived from the dialect of the third tribe, later called the Teleri.

Sindarin Grammar P1: Introduction

Sindarin was originally the native language of the Elves of Beleriand, but by the Third Age it was the language spoken by nearly all the Elves of western Middle Earth. As Tolkien described it in The Lord of the Rings Appendix F:

Of the Eldarin tongues two are found in this book: the High-elven or Quenya, and the Grey-elven or Sindarin ...