Exode 25:16

ונתת אל הארן — It must be explained as though it were written בארון: [AND THOU SHALT PUT] INTO THE ARK, העדות THE TESTIMONY — the Torah (the Tablets; cf. Rashi on Exodus 40:20) which is a testimony between Me and you that I have ordained the commandments written in it (Midrash Tanchuma, Pekudei 4).

וְנָתַתָּ֖ אֶל־הָאָרֹ֑ן אֵ֚ת הָעֵדֻ֔ת אֲשֶׁ֥ר אֶתֵּ֖ן אֵלֶֽיךָ׃

And thou shalt put into the ark the Testimony which I shall give thee.