The Unique Atonement of Shavuos

Beis Aharon - Sefer Shema Shlomo

And one kid of the goats, to make an atonement for you. {28:30} Rabbi Mesharshioh said in the name of Rabbi Eydiy, All [other] cases of the sacrifices, the Torah says, ‘a goat for a sin offering’, but, about Shavuos, it does not mention ‘a sin offering.’ Hakodosh Boruch Hu Said to Klall Yisroel: ‘Since you have accepted upon yourselves the yoke of Torah [on the holiday of Shavuos] I consider it as though you never sinned’.

Talmud Yerushalmi, Rosh Hashonoh 4:8