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
In a healthy body, healthy mind.
To live a life is not the same (not as easy) as to go through a field.
You can't spoil the porridge with butter.
If you like sledding, you should like to carry the sled.
Strike the iron while it's hot.
All is well that ends well.
A well-fed man does not understand a starving one.
Оne can not see the wood for the trees.