- It stands to reason no one has ever seen a white rabbit six feet high.
- Not very often, Doctor.

6 seconds sound clip from the Harvey (1950) movie soundboard.

You can hear this line at 00:37:15.400 in the Blu-ray version of the movie.

Quote context

[...]

- To sum it up, your sister's condition is serious, but I can help her. She must, however, remain out here temporarily.

- Uh-huh.

- Well, I've always wanted Veta to have everything she needs.

- Exactly.

- But I wouldn't want her to stay out here unless she likes it out here and wants to stay here.

- Well, if this were an ordinary delusion, something reflected on the memory picture, that would be one thing.

- But this, this is much more serious.

- It stands to reason no one has ever seen a white rabbit six feet high.

- Not very often, Doctor.

- So she must be committed here temporarily. Under the circumstances, I'd commit my own grandmother.

- Does your grandmother drink, too?

- That's just an expression.

- Now here are the commitment papers for you to sign, and here's a visitor's pass. You can come and see her whenever you like.

[...]