Don't count your chickens before they hatch

Don't make plans based on something good that you expect to happen but that hasn't happened yet; don't assume success before it's certain.

This proverb dates back to ancient times, with versions appearing in Aesop's fables around 600 BCE. The story tells of someone who counts anticipated chickens from eggs not yet hatched, only to lose everything when the eggs break. The English version became popular in the 16th century.