First catch your hare

You cannot cook and eat it until you have caught it, which may not be easy. Don’t decide what you are going to do with a thing until it is in your possession. Act prudently and don’t be over-optimistic.

See also:

• Catch your bear before you sell its skin
• Don't count your chickens before they are hatched
• Do not halloo till you are out of the wood
• Never spend your money before you have it
• There's many a slip 'twixt the cup and the lip