Commentary for I Chronicles 9:33

וְאֵ֣לֶּה הַ֠מְשֹׁרְרִים רָאשֵׁ֨י אָב֧וֹת לַלְוִיִּ֛ם בַּלְּשָׁכֹ֖ת פטירים [פְּטוּרִ֑ים] כִּֽי־יוֹמָ֥ם וָלַ֛יְלָה עֲלֵיהֶ֖ם בַּמְּלָאכָֽה׃

And these are the singers, heads of fathers’houses of the Levites, who dwelt in the chambers and were free from other service; for they were employed in their work day and night.

Rashi on I Chronicles