The planet in question is in fact Magrathea.

3 seconds sound clip from The Hitchhiker's Guide to the Galaxy (1981) - Episode 3 sitcom.

You can hear this line at 00:05:42.888 in the Blu-ray version of the TV Series.

Quote context

[...]

- I still don't believe you.

- Why not?

- You tend to lie a lot.

- Hey!

- I think It's just any old dead planet.

- The suspense is killing me.

- Stress and nervous tension are now serious social problems in all parts of the Galaxy.

- And it is in order that this situation should not be in any way exacerbated that the following facts will now be revealed in advance.

- The planet in question is in fact Magrathea.

- The deadly missile attack shortly to be launched by an ancient automatic defense system will merely result in the breakage of 3 coffee cups, a mouse cage, the bruising of someone's upper arm...

- And the untimely creation and sudden demise of a bowl of petunias, and an innocent sperm whale.

- In order that some sense of mystery should still be preserved, no revelation will yet be made concerning whose upper arm has been bruised.

- This fact may safely be made the subject of suspense, since it is of no significance whatsoever.

[...]