(Devarim 21:5) "Then there shall draw near the Cohanim, the sons of Levi": What is the intent of this? (i.e., Don't we know that the Cohanim are the sons of Levi?) — Because it is written (Ibid.) "for them did the L-rd your G-d choose to serve Him," I might think that only unblemished Cohanim are eligible (for this rite); it is, therefore, written "the sons of Levi" — to teach that blemished Cohanim are eligible for the breaking of the neck of the heifer.