The reading for is the Ten Commandments. This is based on the opinion of one of the Tannaim (early Sages) found in three places in rabbinic literature (Tosefta Megillah 3:5, ed. Lieberman p. 354; Yerushalmi Megillah 3:7, fol. 74b; and Bavli Megillah 31a). This is, without a doubt, the result of the rabbinic belief that the Torah was given on Mt. Sinai on Shavuot ( 86b).
Centrality of the Ten Commandments
Even so, it is very surprising that we only read the Ten Commandments in public on Shavuot and as part of the weekly Torah portions of Yitro (Exodus 20) and Va’ethanan (Deuteronomy 5). After all, the Bible itself considered the Ten Commandments of seminal importance to the covenant between God and the people of Israel. The Ten Commandments are also quoted or paraphrased by the Psalms (50:7, 18-19; 81:10-11), by the prophet Hosea (4:1-2), and by the prophet Jeremiah (7:9).