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
The beauty of a debt is its payment
(lit.) Whoever gets up early, God gives him
If you like sledding, you should like to carry the sled.
If you are chasing two hares at once, you won’t catch a single one.
Don't bring your set of rules in a foreign monastery
Free cheese is only in a mousetrap.
What goes around comes around.
Man does not live thanks to bread alone.