One story reveals that these would have been the words Charlemagne had uttered shortly before his passing from this world; as an answer to inquiry how does it feel to step into the great unknown without any of his trusted men on the side.
Of course, on the other note, we'll have to keep in mind that the authors of many annals aimed to boost the reputation of some legendary characters by creating descriptions that picture the nature of greater entities out of their mind. The case here is a bit similar with Caesar's famous "the dice has been thrown", did he really spell out those words when crossing the Rubicon or is it simply nothing but an early form of a public relations trick?
Another source says that Charlemagne himself was actually quoting somebody, his own father Pippin the Short (alright, Hobbit jokes aside). Perhaps it was some kind of unofficial family motto. Or something.
The cases with many historical quotes dating back ages is that we really don't get to know if the credits have gone right. What we can say for sure, the quote in question is OLD. Reliably, we can say it's most likely mouthed/written by someone living when Charles the Great was around.
Imagine the following situation: Dozens of years after the human race has vanished from planet Earth, a bunch of EBE archaeologists arrive. They find evidence that one day on this rock has lived a man called George W, who's seemingly been some kind of bigwig during his time. They assume all the four docs they've found about this man is fact, even though they're an issue of Washington Times, one of his speeches, a Tom Clancy novel... and a page ripped out of the MAD magazine.
Hmm.