With accent marks:
Оди́н в по́ле не во́ин.
One in the field is not a warrior.
This proverb is used to emphasize that only acting together people represent a force that can overcome difficulties.
However, there is also an opposite proverb in Russian:
И один в поле воин.
Even one is a warrior in the field.
This means that there are situations when a lot depends on one person.
In European languages, there are similar proverbs that speak of the limited abilities of an individual. The closes equivalent in English: there is safety in numbers.
Other Russian proverbs
A wolf is fed by his legs
Live for a century, learn for a century, but die being a fool anyway.
The quieter you go, the further you'll get.
The eyes are scared, but the hands are doing.
Don't go to the woods if afraid of wolves.
A titmouse in hands is better than a crane in the sky.
Repetition is the mother of learning.
If you like sledding, you should like to carry the sled.