count one's chickens before they hatch
Make plans based on events that may or may not happen. For example, You might not win the prize and you've already spent the money? Don't count your chickens before they hatch! or I know you have big plans for your consulting business, but don't count your chickens. This expression comes from Aesop's fable about a milkmaid carrying a full pail on her head. She daydreams about buying chickens with the milk's proceeds and becoming so rich from selling eggs that she will toss her head at suitors; she then tosses her head and spills the milk. Widely translated from the original Greek, the story was the source of a proverb and was used figuratively by the 16th century. Today it is still so well known that it often appears shortened and usually in negative cautionary form (don't count your chickens).