Or HaChaim on Numbers 19:3/אור החיים על במדבר י״ט:ג׳

1 ונתתם אתה אל אלעזר הכהן, "and you are to hand it over to Eleazar the priest, etc." The restrictive word אתה, "it" is necessary in view of a ruling in Yuma 42 that we have a tradition according to which subsequent red heifers could be handled either by the High Priest or by an ordinary priest, whereas in this instance it had to be handled by the High Priest.