Exodus 26:34
"And thou shalt put the mercy seat upon the ark of the testimony in the most holy place."
Cross References (ranked by relevance)
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 upon the ark:
Exodus 25:21
And thou shalt put the mercy seat above upon the ark; and in the ark thou shalt put the testimony that I shall give thee.
Hebrews 9:5
And over it the cherubims of glory shadowing the mercyseat; of which we cannot now speak particularly.