Nehemiah 13:30 Midrash

וְטִֽהַרְתִּ֖ים מִכָּל־נֵכָ֑ר וָאַעֲמִ֧ידָה מִשְׁמָר֛וֹת לַכֹּהֲנִ֥ים וְלַלְוִיִּ֖ם אִ֥ישׁ בִּמְלַאכְתּֽוֹ׃

Thus cleansed I them from everything foreign, and appointed wards for the priests and for the Levites, every one in his work;