This is the statute of the Torah that the Lord has commanded, saying: Speak to the children of Yisrael, and they shall take to you a red heifer, perfect, in which there is no blemish, and upon which a yoke has not been laid.
Torah Temimah: Yoma 2a — Seven days before the burning of the para they separate the Kohen who burns it from his house. Source: “asher tzivah Hashem” here and “ka’asher asah bayom hazeh tzivah Hashem la’asos” there (Tzav); just as there separation, so here separation.
English: We learned there: Seven days before the burning of the heifer they separate the Kohen who burns it from his house. From where [do we know this]? It is written here “which the Lord commanded” and it is written there “as he did on this day, the Lord commanded to do”; just as there separation, so here separation.
Gezeirah shavah on “tzivah Hashem.”
The Kohen designated to burn the para aduma is sequestered for seven days beforehand, parallel to the isolation of the Kohen Gadol before Yom Kippur.