דברים יח:ו מדרש: Sifrei Devarim

(Devarim 18:6) "And if the Levite come": I might think that Scripture speaks of a Levite per se; it is, therefore, written (7) "And he shall serve" — (Scripture speaks of) those who are qualified to serve (i.e., the Cohanim, who perform the sacrificial service), to exclude the Levites, who are not thus qualified.