וַיַּעֲמִידֵ֣ם לָעַ֣ד לְעוֹלָ֑ם חָק־נָ֝תַ֗ן וְלֹ֣א יַעֲבֽוֹר׃
Er gab ihnen für ewig Bestand; ein Gesetz gab er und es wandelt nicht.
Rashi on Psalms
He issued a decree to them, that this one should serve by day and that one should serve by night. which will not change That decree.