With his ducal signet ring, the Duke put the seal of the House of Atreides under his signature, Duke Leto of Arrakis.
10 seconds sound clip from the Dune (1984) movie soundboard.
You can hear this line at 00:43:33 in the Blu-ray version of the movie.
Quote context
[...]
- This then, is the Duke's response as he sat writing at his desk:
- Your offer of a meeting is refused.
- I have ofttimes met your treachery, and this all men know.
- The art of Kanly still has its admirers in the empire.
- Then he signed it Duke Leto of Arrakis knowing full well that this would annoy the Baron.
- Vas!
- Soon the Baron would have his answer.
- Among those who serve the Baron is Piter De Vries,
[...]