Parashas Bamidbar · Passuk 3:38
Those who were to camp before the Tabernacle, in front—before the Tent of Meeting, on the east—were Moses and Aaron and his sons, attending to the duties of the sanctuary, as a duty on behalf of the Israelites; and any outsider who encroached was to be put to death.
Chabad.org — English translation + Rashi
Source: Tamid 26b.
English: “Keeping the charge of the sanctuary” — we learned in a Mishnah: The kohanim keep watch in three places in the Temple.
In context: The verse is the source for the institution of the Temple watches.
Custom
Chazal derive from this passuk the three stations at which the kohanim stood guard in the Temple.