Client wants artwork for reprint purposes

Recently I did some leaflet design work for a client, along with quote for and arrangement of printing.
I added the usual 10% commission on printing.

Now client is asking that I send him the artwork files in case he wants more printing.

Am I correct in saying that the artwork belongs to me, and that I have the right to retain it, and he has to go through me if he wants further print runs?

And how best to put it to him?

thanks