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
Another's soul is darkness.
In a healthy body, healthy mind.
Hurry to much and you will make people laugh.
A bad example is contagious.
You can't command to the heart.
Appetite comes with eating.
The demon is not as scary as he is depicted.
In a quiet whirlpool, demons live.