Rashi on Terumah 25:16

English: 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).