There are a few things that could apply here.
You could be looking for the word consensus:
a : general agreement : unanimity
b : the judgment arrived at by most of those concerned
: group solidarity in sentiment and belief
Or you could be looking for the logical fallacy Argumentum ad populum:
Argumentum ad populum
In logic, an argumentum ad populum (Latin for "appeal to the people") is a fallacious argument that concludes a proposition to be true because many or most people believe it. In other words, the basic idea of the argument is: "If many believe so, it is so."
This type of argument is known by several names,1 including appeal to the masses, appeal to belief, appeal to the majority, appeal to democracy, argument by consensus, consensus fallacy, authority of the many, and bandwagon fallacy, and in Latin as argumentum ad numerum ("appeal to the number"), and consensus gentium ("agreement of the clans"). It is also the basis of a number of social phenomena, including communal reinforcement and the bandwagon effect. The Chinese proverb "three men make a tiger" concerns the same idea.
You could also find the word or phrase you're looking for in that definition.