He's the liar.
4 seconds sound clip from the Labyrinth (1986) movie soundboard.
You can hear this line at 00:27:58 in the Blu-ray version of the movie.
Quote context
[...]
- And I should warn you that one of us always tells the truth, and one of us always lies.
- All right. Answer yes or no.
- Would he tell me that this door leads to the castle?
- Yes.
[...]