This dates back to at least 1949, where it is referenced in these conference proceedings with attribution to Carl Sandburg (1878 – 1967):
I've also heard it as "all of us are better than one of us", which dates at from the late nineties.
Edit: Carl Sandburg's original line is "Everybody is cleverer than anybody", published in 1936's The People, Yes. (You can read a bit more of it in this book.)
Edit 2: And "everybody is cleverer than anybody" goes to back 1877, where it is referenced by several Westminster publications as an "epigram of Talleyrand" or "Talleyrand's famous sentence". This is the French diplomat Charles Maurice de Talleyrand-Périgord (1754 – 1838). I've not found the original French, but it has also been translated as:
“There is one body that knows more than anybody, and that is everybody”