1 Kings 8:9, 21

There was nothing in the ark except the two tablets of stone which Moses put there at Horeb (Sinai), where the LORD made a covenant with the Israelites when they came out of the land of Egypt. [Deut 10:2-5]

There I have made a place [in the Holy of Holies] for the ark, in which is the covenant (solemn agreement) of the LORD, which He made with our fathers when He brought them out of the land of Egypt.” [Ex 34:28]