With accent marks:
Запре́тный плод сла́док.
The forbidden fruit is sweet.
Similar proverbs are found in many languages of the world. They all originate from the Bible.
Nowadays, the expression is used as a warning to those who like to enjoy harmful activities: smoking, drinking alcohol, etc.
Other Russian proverbs
They do not seek good from good.
Strike the iron while it's hot.
You can't hide an awl in a bag.
One in the field is not a warrior.
Don’t wave your fists after the fight.
A holy place is never empty.
Beast runs towards the hunter.
You see your friends in the times of trouble.