"Well, that means I won't see much of Professor Snape from now on," he said, "because he won't let me carry on Potions unless I get 'Outstanding' in my OWL., which I know I haven't."
"Don't count your owls before they are delivered," said Dumbledore gravely. "Which, now I think of it, ought to be some time later today. Now, two more things, Harry, before we part." (HBP4)
Definition
Wizarding equivalent of the phrase “Don’t count your chickens before they hatch,” meaning don’t assume that you know what the outcome of the situation will be.
Dumbledore uses this phrase to caution Harry that he might be mistaken when he assumes that he won't be able to take Potions during the upcoming school year because Snape won't allow anyone in his NEWT classes without high marks. Dumbledore knows what Harry does not, that Slughorn will be teaching Potions, not Snape.
Commentary
Etymology
A wizarding version of "don't count your chickens before they hatch."