Num Numbers, chapter Book of Numbers, chapter 8
And I have given the Levi'im as a gift to Aharon and to his Banim from among the Bnei Yisroel, to do the Avodat Bnei Yisroel in the Ohel Mo'ed, and to make kapporah for the Bnei Yisroel; so that there be no plague among the Bnei Yisroel, when the Bnei Yisroel come near unto HaKodesh.Num 3:9