Sforno on Deuteronomy 31:26/ספורנו על דברים ל״א:כ״ו

1והיה שם בך לעד, the written Torah, reposing in the Holy Ark is witness that I have predicted and warned against your abandoning G’d and His Torah. This is why I arranged for this testimony to be preserved in a place accessible even to the High Priest only once a year on the Day of Atonement. This original copy of the written Torah will, if need be, prove that all that is written in all the Torah scrolls later and accessible to all the people all the time was a true copy of the Torah Moses had received from G’d at Mount Sinai. It will serve as proof that the Torah scrolls in circulation among the Torah scholars and the righteous people who commissioned copies for their private libraries, are all true to the original and do not contain additions or omissions. It would appear, however, that the Torah scroll found by the High Priest Chilkiyah (Kings II 22,8) was the very one which Moses had handed over to the priests who carried the Ark across the river Jordan which we referred to earlier. It contained only the passage dealing with the legislation pertaining to the king. It was this scroll in which Joshua wrote (added) the renewal of G’d’s covenant with the people which took place at Shechem. This was the record of Israel’s renewed undertaking to serve the Lord. Compare Kings II 22,13 where King Yoshiyahu refers to this scroll containing matters not contained in Moses’ original Torah scroll, as Joshua had written part of this scroll after the covenant’s renewal in Shechem. The King had become painfully aware how far the people had strayed from their solemn undertaking.