Idiom/proverb: if you keep calling someone a mouse, they might start thinking they're a mouse

I know there is a similar idiom in English and in other languages like Chinese languages, but I can't remember the exact idiom.

It's something like "if you keep calling an elephant a mouse, it might start thinking it's a mouse."

The meaning is if you keep telling a person they're stupid, one day they might actually believe it and think they're stupid.

Please help.

In sociology, the phenomenon described is referred to as the "Thomas theorem". Even though it may not pass as an idiom per se, its originators Thomas and Thomas summarize the theorem like this:

"If men define situations as real, they are real in their consequences"

(The child in America: Behavior problems and programs. W.I. Thomas and D.S. Thomas. New York: Knopf, 1928: 571–572)

In education circles, it's called Pygmalion effect.

Pygmalion effect: The Pygmalion effect, or Rosenthal effect, is the phenomenon whereby others' expectations of a target person affect the target person's performance.

[Wikipedia]