וְ֠הָיָה כִּֽי־תִמְצֶ֨אןָ אֹת֜וֹ רָע֣וֹת רַבּוֹת֮ וְצָרוֹת֒ וְ֠עָנְתָה הַשִּׁירָ֨ה הַזֹּ֤את לְפָנָיו֙ לְעֵ֔ד כִּ֛י לֹ֥א תִשָּׁכַ֖ח מִפִּ֣י זַרְע֑וֹ כִּ֧י יָדַ֣עְתִּי אֶת־יִצְר֗וֹ אֲשֶׁ֨ר ה֤וּא עֹשֶׂה֙ הַיּ֔וֹם בְּטֶ֣רֶם אֲבִיאֶ֔נּוּ אֶל־הָאָ֖רֶץ אֲשֶׁ֥ר נִשְׁבָּֽעְתִּי׃
then it shall come to pass, when many evils and troubles are come upon them, that this song shall testify before them as a witness; for it shall not be forgotten out of the mouths of their seed; for I know their imagination how they do even now, before I have brought them into the land which I swore.’
Read halakhic rulings (Jewish law) on Deuteronomy 31:21: and classical halakhic sources.