Chabad.org — English translation + Rashi
Torah Temimah points to Toras Kohanim (Sifra), on 4:4 (hevi et ha-par — the bull itself, not a substitute).
Sefaria — Sifra / Toras Kohanim
English: “And he shall bring the bull” — this teaches that it is a bull that he brings, and not another offering as its substitute.
Miyut of substitute offering — the designated bull must be brought; one may not substitute a different animal.
The designated bull must be brought; one may not substitute a different animal. The Sifra’s reading of “and he shall bring the bull” teaches that the specific animal designated for the High Priest’s chatas is required. A different offering cannot serve as a substitute, preserving the integrity of the animal designated for this unique chatas.