낮말은 새가 듣고 밤말은 쥐가 듣는다

Literal Meaning

Birds hear the words spoken in the day, and the mice those at night.

Usage

Used to warn someone to be careful about what he or she says since one never knows who may be listening.

Korean Equivalent

비밀로 한 일도 남들이 알게 된다.

English Equivalent

Walls have ears.