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 27a.
English: “Keeping the charge” — \ldots from where is this derived? Rav Ashi said: from the verse “and those who encamp before the Mishkan.”
In context: The camping description itself is read as the source for the permanent watch system.
Custom
Chazal locate the scriptural basis for the priestly watches in the camping arrangement around the Mishkan.