Learn English – How to express high proficiency in a succinct way

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”

It’s certainly found in the speech and writing of native speakers, but it’s a cliché, and you might want to avoid it for that reason alone, whether you’re a native speaker or not. It’s heard in the United Kingdom, but its origin is probably American. The earliest citation I’ve been able to track down in the OED is this from the American writer GB Trudeau in 1980: ‘Seriously, I think I know where you're coming from, and I'd like to share that space.’