Sifrei Bamidbar 67

(Bamidbar 9:5) "And they offered the Pesach in the first (month) on the fourteenth day of the month": Scripture speaks in disparagement of Israel, that all the forty years that they were in the desert they offered only this one Paschal sacrifice. And thus is it written (Amos 5:25) "Did you bring sacrifices and meal-offerings to Me for forty years in the desert?" R. Shimon b. Yochai says: Israel did not sacrifice, and who did sacrifice? The tribe of Levi, as it is written (Devarim 33:10) "They shall place incense before You and a burnt-offering upon Your altar." Israel served idolatry and the Levites did not serve idolatry, as it is written (Ibid. 9) "For they kept Your commandment ("You shall have no other gods"). And it is written (Shemot 32:26) "And Moses stood in the gate of the camp, and he said 'Whoever is for the L-rd, (let him come) to me!' And there gathered unto him all the sons of Levi." Israel did not circumcise themselves (in the desert), as it is written (Joshua 5:5) "and all the people who were born in the desert … were not circumcised," but the Levites were circumcised, viz. (Devarim 33:10) "and Your covenant (of circumcision) they kept." (Bamidbar 9:5) "According to all that the L-rd had commanded Moses": to declare the praise of Israel. Just as Moses told them, "thus did they do."