Shavuous as Time of giving Torah not mentioned in Torah

Shemos (23:16) and the Feast of the Harvest (Shavuos), of the first fruits of your work, of what you sow in the field; and the Feast of Ingathering at the end of the year, when you gather in the results of your work from the field.

Shemos (34:22) You shall observe Shavuos, of the first fruits of the wheat harvest; and the Feast of Ingathering at the turn of the year.

Bamidbar (28:26) On the day of the first fruits, your Feast of Weeks (Shavuos), when you bring an offering of new grain to G-d, you shall observe a sacred occasion: you shall not work at your occupations.

Devarim (16:10) Then you shall observe Feast of Weeks (Shavuos) for your G-d, offering your freewill contribution according as your G-d has blessed you.

Pesachim (68b) Shavuos is referred to as atzeres