Rashi on Exodus 31:7/רש"י על שמות ל״א:ז׳

1ואת הארן לעדת AND THE ARK FOR THE TESTIMONY — for the purpose of the Tablets of the testimony (not, “the ark to serve as the testimony”; the ל denotes “for” in the sense of “for the use of”, and the term עדת is an abbreviation of לחות העדות).