Exodus 25:21
And you shall put the mercy seat above on the ark; and in the ark you shall put the testimony that I shall give you.
Torrey's Topical Textbook—AKJV

Exodus 26:34
And you shall put the mercy seat on the ark of the testimony in the most holy place.
Torrey's Topical Textbook—AKJV

Exodus 40:20
And he took and put the testimony into the ark, and set the staves on the ark, and put the mercy seat above on the ark:
Torrey's Topical Textbook—AKJV