Chabad.org — English translation + Rashi
Torah Temimah points to Zevachim 63a.
E-Daf.com — Zevachim 63a
HebrewBooks / Shas PDF
English: Rabbi Yochanan said: Shelamim slaughtered inside the heichal are valid, as it is said “at the entrance of the Tent of Meeting” — and the secondary should not be stricter than the primary.
Validity of shechitah inside the heichal — slaughter inside the Sanctuary is no worse than at its entrance.
Slaughter inside the Sanctuary is no worse than at its entrance. Rabbi Yochanan reasons that the secondary location (inside) should not be held to a stricter standard than the primary location named in the verse (the entrance). Therefore shelamim slaughtered within the heichal itself remain valid.