TSK · Exodus 25:21

You are to put into the ark the testimony that I will give to you.

He must then put the incense on the fire before the LORD, and the cloud of incense will cover the atonement lid which is above the ark of the testimony, so that he will not die.

There was nothing in the ark except the two stone tablets Moses had placed there in Horeb. It was there that the LORD made a covenant with the Israelites after he brought them out of the land of Egypt.