There is no secret ingredient.
5 seconds sound clip from the Kung Fu Panda movie soundboard.
You can hear this line at 01:09:40 in the Blu-ray version of the movie.
Quote context
[...]
- The secret ingredient of my Secret Ingredient Soup.
- Come here. The secret ingredient is nothing. You heard me. Nothing! There is no secret ingredient!
- Wait, wait. It's just plain old noodle soup? You don't add some kind of special sauce or something?
- To make something special, you just have to believe it's special.
- There is no secret ingredient.
- This is no longer your home. And I am no longer your master.
- Oh, yes. You have a new favorite.
- So where is this Po? Did I scare him off?
- This battle is between you and me.
[...]