don’t count your chickens before they hatch

Don’t spend or try to profit from something not yet earned. This expression comes from Aesop’s fable about a milkmaid carrying a full pail on her head who daydreams about selling the milk for eggs that will hatch into chickens and make her so rich she will toss her head at offers of marriage; but she prematurely tosses her head and spills the milk. It was, like so many Greek fables, translated into modern European languages and passed on. The expression was in use figuratively by the sixteenth century and appeared in proverb collections soon afterward.