וְעַתָּ֗ה כִּתְב֤וּ לָכֶם֙ אֶת־הַשִּׁירָ֣ה הַזֹּ֔את וְלַמְּדָ֥הּ אֶת־בְּנֵי־יִשְׂרָאֵ֖ל שִׂימָ֣הּ בְּפִיהֶ֑ם לְמַ֨עַן תִּהְיֶה־לִּ֜י הַשִּׁירָ֥ה הַזֹּ֛את לְעֵ֖ד בִּבְנֵ֥י יִשְׂרָאֵֽל׃
Now therefore write ye this song for you, and teach thou it the children of Israel; put it in their mouths, that this song may be a witness for Me against the children of Israel.
Read kabbalistic (mystical) interpretation of Deuteronomy 31:19: and classical mystical sources.