וְנָתַתָּ֧ אֶת־הַכַּפֹּ֛רֶת עַל־הָאָרֹ֖ן מִלְמָ֑עְלָה וְאֶל־הָ֣אָרֹ֔ן תִּתֵּן֙ אֶת־הָ֣עֵדֻ֔ת אֲשֶׁ֥ר אֶתֵּ֖ן אֵלֶֽיךָ׃
And thou shalt put the ark-cover above upon the ark; and in the ark thou shalt put the testimony that I shall give thee.
Read kabbalistic (mystical) interpretation of Exodus 25:21: and classical mystical sources.