Sifrei_mitzvot for Numbers 8:25

וּמִבֶּן֙ חֲמִשִּׁ֣ים שָׁנָ֔ה יָשׁ֖וּב מִצְּבָ֣א הָעֲבֹדָ֑ה וְלֹ֥א יַעֲבֹ֖ד עֽוֹד׃

and from the age of fifty years they shall return from the service of the work, and shall serve no more;