TSK · Exodus 25:21

You shall put the testimony which I shall give you into the ark.

and he shall put the incense on the fire before Yahweh, that the cloud of the incense may cover the mercy seat that is on the testimony, so that he will not die.

There was nothing in the ark except the two tables of stone which Moses put there at Horeb, when Yahweh made a covenant with the children of Israel, when they came out of the land of Egypt.