Sifrei Devarim 208

¹(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. ²"to serve Him and to pronounce the benediction in the name of the L-rd": The (priestly) benediction is being likened to the (priestly) service. Just as the latter is performed, standing, so, the former. ³"and by their word shall every contention and every plague-spot be (ruled upon."): Contentions are being compared to plague-spots, and plague-spots to contentions. Just as plague-spots are (ruled upon) only in the daytime, so, contentions. And just as contentions are not (ruled upon) by kin, so, plague-spots. If so, why not say: Just as contentions require three (judges), so, plague-spots. ⁴And it would follow a fortiori , viz.: If his money requires three, how much more so, his body! It is, therefore, written (re plague-spots, Vayikra 13:3) "… or to one of his sons, the Cohanim" — One Cohein rules on a plague-spot.