Parashas Bamidbar
וְהַחֹנִ֣ים לִפְנֵ֣י הַמִּשְׁכָּ֡ן קֵ֣דְמָה לִפְנֵי֩ אֹֽהֶל־מוֹעֵ֨ד ׀ מִזְרָ֜חָה מֹשֶׁ֣ה ׀ וְאַהֲרֹ֣ן וּבָנָ֗יו שֹֽׁמְרִים֙ מִשְׁמֶ֣רֶת הַמִּקְדָּ֔שׁ לְמִשְׁמֶ֖רֶת בְּנֵ֣י יִשְׂרָאֵ֑ל וְהַזָּ֥ר הַקָּרֵ֖ב יוּמָֽת׃
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.
Torah Temimah points to TT (second unit) (verified).
שמרים משמרת —\ldots מנא הני מילי, אמר רב אשי, דאמר קרא והחונים לפני המשכן.
English: “Keeping the charge” — \ldots from where is this derived? Rav Ashi said: from the verse “and those who encamp before the Mishkan.”
Custom
Chazal locate the scriptural basis for the priestly watches in the camping arrangement around the Mishkan.