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
Bitter truth is better than sweet lies.
Everything has its time.
Where there is no fish even a crayfish is a fish.
Better late than never.
Better to see once than hear a hundred times.
Each sandpiper praises its swamp.
You see your friends in the times of trouble.
Forewarned is forearmed.