C. F. Keil and Franz Delitzsch on 1 Kings 8:9

1Ki 8:9 “There was nothing in the ark but the two tables of stone, which Moses had put there at Horeb, when Jehovah concluded the covenant with Israel.” The intention of this remark is also simply to show that the law, which enjoined that the ark should merely preserve the stone tables of the covenant (Exo 25:16; Exo 40:20), had not been departed from in the lapse of time. אשׁר before כּרת is not a pronoun, but a conjunction: when, from the time that, as in Deu 11:6, etc. כּרת without בּרית, signifying the conclusion of a covenant, as in 1Sa 20:16; 1Sa 22:8, etc. Horeb, the general name for the place where the law was given, instead of the more definite name Sinai, as in Deuteronomy (see the Comm. on Exo 19:1-2).The statement in Heb 9:4, to the effect that the pot of manna and Aaron’s rod that budded were also to be found in the ark, which is at variance with this verse, and which the earlier commentators endeavoured to bring into harmony with it by forced methods of different kinds, simply rests upon an erroneous interpretation of העדוּת לפני in Exo 16:33-34, and Num 17:10, which had become traditional among the Jews; since this merely affirms that the objects mentioned had been deposited in front of the testimony, i.e., in front of the ark which contained the testimony, and not within it, as the Jews supposed. - Still less are De Wette and others warranted in deducing from this verse an argument against the existence of the Mosaic book of the law in the time of Solomon, inasmuch as, according to the precept in Deu 31:26, the book of the law was not to be kept in the ark, but by the side of it, or near it.