If fifty million people say a foolish thing, it is still a foolish thing.
-Anatole France
So this quote is commonly, but falsely, attributed to Bertrand Russell.
Wikiquote attributes this to Anatole France, but I’m suspicious of the cited source. Specifically, they reference the quote as appearing in 1954’s Listening and Speaking: A Guide to Effective Oral Communication by Ralph G. Nichols and Thomas R. Lewis.
Aside from making that a second hand rather than primary source, the Wikiquote page notes that a virtually identical saying appears in W. Somerset Maugham’s A Writer’s Notebook. That book was published in 1949, but contains entries from a journal that Maugham began keeping in 1892.
According to Wikiquote, the following quote appears in a 1901 entry in A Writer’s Notebook,
If forty million people say a foolish thing it does not become a wise one, but the wise man is foolish to give them the lie.