Rashi on Leviticus 8:34/רש"י על ויקרא ח׳:ל״ד

1צוה ה' לעשת [AS HE HATH DONE THIS DAY, SO] THE LORD HATH COMMANDED TO DO all the seven days). But our Rabbis explained as follows: לעשת, “so hath the Lord commanded to do” — this refers to the “Red Heifer” (Numbers ch. XIX.); and לכפר, “to perform a cleansing ceremony” — this refers to Yom-Kippur. And thus these words, they say, are intended to intimate that the High Priest requires separating from his family during seven days before Yom-Kippur, and similarly the priest who burns the Red Heifer (Sifra, Tzav, Mechilta d'Miluim 1 37; Yoma 2a; Yoma 3b).