Shemot 40:20

20 And he took and placed HaEdut (The Testimony, i.e., the Stone Tablets of the Ten Commandments) into HaAron, and set the carrying poles on the Aron, and put the Kapporet upon the Aron from above;

Shemot 40:20 Meaning and Commentary

Exodus 40:20

And he took and put the testimony into the ark
The ten commands, called the "testimony", because they testified and declared what was the will of God with respect to things to be done, or not done. The Targum of Jonathan says, he put the broken pieces of the tables into it; but what became of them, or were done with them, we nowhere read; and it does not appear that any other, but those that Moses hewed by the order of God, were put into the ark; see ( Deuteronomy 10:1-5 )

and set the staves on the ark;
that it might be ready to be carried whenever a remove was necessary:

and put the mercy seat above upon the ark;
together with the cherubim.

Shemot 40:20 In-Context

18 And Moshe erected the Mishkan, and fastened its sockets, and set up the kerashim thereof, and put in the crossbars thereof, and set up its ammudim.

19 And he spread the Ohel over the Mishkan, and put the Mikhseh of the Ohel on it from above; as Hashem commanded Moshe.

20 And he took and placed HaEdut (The Testimony, i.e., the Stone Tablets of the Ten Commandments) into HaAron, and set the carrying poles on the Aron, and put the Kapporet upon the Aron from above;

21 And he brought the Aron into the Mishkan, and set up the Parochet HaMasach, and shielded with a covering over the Aron HaEdut; as Hashem commanded Moshe.

22 And he put the Shulchan in the Ohel Mo’ed, upon the north side of the Mishkan, outside the Parochet.