Rashi on Exodus 25:16/רש"י על שמות כ״ה:ט״ז

1ונתת אל הארן — It must be explained as though it were written בארון: [AND THOU SHALT PUT] INTO THE ARK,

2העדות 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).