Don't count your chickens before they hatch
laː tuʕidː ʔaddːdʒaːdʒa qabl ʔn jfqs
In Arabic لا تُعِدَّ الدَّجاجَ قبل أنْ يفقس.
This proverb is a direct equivalent, warning against counting on something before it's certain.
Example
"كان يحتفل بالفعل، لكنني قلت له: 'لا تُعِدَّ الدَّجاجَ قبل أنْ يفقس.'"
