Don't count your chickens before they hatch
Literally 'don't sell the bear's skin before hunting it'. Warns against celebrating or planning around something that hasn't happened yet.
Don't celebrate yet, don't count your chickens before they hatch.
—I'm already thinking about how to spend the money. —Don't count your chickens before they hatch.
It's a good project, but don't count your chickens before they hatch.