You shall put the mercy seat on top of the ark, and in the ark you shall put the Testimony that I will give you.
Exodus 25:21
Surrounding context
25:20 And the cherubim shall stretch out their wings above, covering the mercy seat with their wings, and they shall face one another; the faces of the cherubim shall be toward the mercy seat.
25:21 You shall put the mercy seat on top of the ark, and in the ark you shall put the Testimony that I will give you.
25:22 And there I will meet with you, and I will speak with you from above the mercy seat, from between the two cherubim which are on the ark of the Testimony, about everything which I will give you in commandment to the children of Israel.