The trouble's not in the mirror, it's in my mind. It must be. A mirror's just wood and glass.
6 seconds sound clip from the Dead of Night (1945) movie soundboard.
You can hear this line at 00:39:41.458 in the Blu-ray version of the movie.
Quote context
[...]
- The only thing to do is to try not to look in it at all.
- But in a queer sort of way it fascinates me.
- The trouble's not in the mirror, it's in my mind. It must be. A mirror's just wood and glass.
- Peter, I don't know what to say. Perhaps you're overworked. Why don't you see a doctor?
- I have. He couldn't find anything wrong with me.
- I think I shall have to go see a mental specialist.
- Oh, nonsense! You're as sane as I am.
- Listen I've been putting off saying this, but I think we ought to postpone the wedding.
- That's a bit drastic, isn't it?
[...]

 Dead of Night (1945)
 Dead of Night (1945)  Phantom of the Opera (1943)
 Phantom of the Opera (1943)  Scooby-Doo, Where Are You! (1969)
 Scooby-Doo, Where Are You! (1969)  Halloween III: Season of the Witch (1982)
 Halloween III: Season of the Witch (1982)  Knight Rider (1982)
 Knight Rider (1982)