Don’t count your chickens before they hatch

This everyday expression means that if you act as if you already have a possession before you own it, you may come unstuck.
The expression comes from one of Aesop's Fables, ‘The Milkmaid and her Pail’. Aesop lived between c. 620-560 BCE.

A young milkmaid was walking to market with a pail of milk to sell, balanced on her head. She started to dream about what she could do with the money once the milk was sold.
She day-dreamed of buying chickens, and then selling their eggs. With that money she could buy clothes to impress the boys. This would make her friends jealous, and to show her indifference, she dreamed she would toss her hair as if she didn’t care.
Unfortunately, she did toss her hair, the milk pail fell off her head, and spilt all the milk on the ground. She had to return home empty handed and explain what happened to her mother.
Her mother listened to the story then told her daughter not to count her chickens before they were hatched.