Chabad.org — English translation + Rashi
Torah Temimah points to Zevachim 55b.
E-Daf.com — Zevachim 55b
HebrewBooks / Shas PDF
English: Rav Yehudah said in the name of Shmuel: Shelamim slaughtered before the heichal doors are opened are invalid, as it is said “and he shall slaughter it at the entrance of the Tent of Meeting” — when it is open, and not when it is locked.
Diyuk of petach as open entrance — the entrance must be open at the time of slaughter.
The entrance must be open at the time of slaughter. Shmuel’s ruling derives from the verse that shelamim slaughtered while the heichal doors are still locked are invalid. The diyuk turns on the ordinary sense of “entrance”: an entrance that is locked is not functioning as an entrance.