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 Mishnah / TT (verified).
שמרים משמרת המקדש — תנן, בשלשה מקומות הכהנים שומרים בבית המקדש.
English: “Keeping the charge of the sanctuary” — we learned in a Mishnah: The kohanim keep watch in three places in the Temple.
Custom
Chazal derive from this passuk the three stations at which the kohanim stood guard in the Temple.