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
(lit.) Whoever gets up early, God gives him
Strike the iron while it's hot.
Without labor, you can not even catch a fish in the pond.
A bird is visible by its flight.
We got what we fought for.
Silent means consent.
Man does not live thanks to bread alone.
The demon is not as scary as he is depicted.