밤말은 쥐가 듣고 낮말은 새가 듣는다

Etymology
Native Korean, literally meaning "rats hear nighttime talks and birds hear daytime talks"

Proverb

 * 1) walls have ears

Usage notes

 * The order of bam "nighttime" before nat "daytime" accords with "night and day" and  "yin and yang, dark and light," better than the alternative form vice versa.