Divrei Hayamim 52:4

זֶ֥ה הַדָּבָ֖ר אֲשֶׁ֣ר תַּעֲשׂ֑וּ הַשְּׁלִשִׁ֨ית מִכֶּ֜ם בָּאֵ֣י הַשַּׁבָּ֗ת לַכֹּֽהֲנִים֙ וְלַלְוִיִּ֔ם לְשֹׁעֲרֵ֖י הַסִּפִּֽים׃

This is the thing that you shall do; A third part of you who come in on the sabbath, of the priests and of the Levites, shall be on duty at the gates;