Reference for Leviticus 16:34

וְהָֽיְתָה־זֹּ֨את לָכֶ֜ם לְחֻקַּ֣ת עוֹלָ֗ם לְכַפֵּ֞ר עַל־בְּנֵ֤י יִשְׂרָאֵל֙ מִכָּל־חַטֹּאתָ֔ם אַחַ֖ת בַּשָּׁנָ֑ה וַיַּ֕עַשׂ כַּאֲשֶׁ֛ר צִוָּ֥ה יְהוָ֖ה אֶת־מֹשֶֽׁה׃ (פ)

And this shall be an everlasting statute unto you, to make atonement for the children of Israel because of all their sins once in the year.’ And he did as the LORD commanded Moses.