You're joking, of course.

4 seconds sound clip from the Mr. Topaze (1961) movie soundboard.

You can hear this line at 01:05:19 in the Blu-ray version of the movie.

Quote context

[...]

- I even know someone who would be sitting in your chair if he'd been willing to work at a cut-price like you.

- Like me, at a cut-price?

- Oh! That's a good one. That is a good one.

- Just between ourselves, how much are you getting out of it?

- 2500 francs. (2)

- You're joking, of course.

- I am not joking.

- My dear sir! I agree that in business it's often a good thing to pretend to be stupid, but really, I think you're carrying it a little far.

- That was a rude and untimely remark.

- It would do well if you were to reflect a little before speaking.

[...]