Charles Ellicott Commentary 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 40:20 (ASV)

He took and put the testimony into the ark. By “the testimony” we must understand the two tables of stone, written with the finger of God, which Moses had brought down with him from the top of Sinai on his last visit (Exodus 34:29). On the use of the word “testimony” in this sense, see Note to Exodus 25:16. It is not proved by this passage that the Ark held nothing but “the testimony.”

Set the staves on the ark: i.e., placed them in the rings, ready for use .