When did the tradition of interpreting Shavuot as commemorating the giving of the Torah ("Zman matan toratenu") begin and what is the first source for that?
In the Torah itself, Shavuot is called the Feast of the Harvest (חגהקציר, Chag HaKatzir)[Exodus 23:16]; the Feast of Weeks (חַ֤גשָׁבֻעֹת֙)[Exodus 34:22, Leviticus 23:15-21, and Deut. 16:9–11]; and the Feast the First Fruits (יוםהבכורים) [Numbers 28:26] Nothing about the giving of the Torah.
Edit: The proposed answer, Where in Tanach is Shavuot associated with Matan Torah?, does not answer my question. What is the earliest source that interprets Shavuot as commemorating the giving of the Torah? (For openers, where in the Talmud is this interpretation?)