It's very rude to sit down without being invited.
4 seconds sound clip from the Alice in Wonderland (1951) movie soundboard.
You can hear this line at 00:42:37 in the Blu-ray version of the movie.
Quote context
[...]
- A very merry unbirthday to you. Who, me?
- Let's all congratulate us with another cup of tea.
- A very merry unbirthday to you.
(BOTH) No room. No room. No room.
- I thought there was plenty of room.
- It's very rude to sit down without being invited.
- I'll say it's rude. It's very, very rude indeed.
- Very, very, very rude indeed.
- I'm very sorry. But I did enjoy your singing.
- I wonder if you could tell me...
[...]