Charlemagne quote?

This has probably been asked Ad Nauseum, but I couldn't find it myself. In "The Last Crusade", Henry Jones Sr. (Sean Connery) says "I suddenly remembered my Charlemagne". Which quote is that, and what is the text's name that I can find this in full?

 

"I suddenly remembered my Charlemagne: Let my armies be the rocks and the trees, and the birds in the sky"

Wikipedia for Charlemangne.
As far as I can make out, this is just a quote attributed to him and not part of a book/poem.

Another interesting note about Charlemagne was that he took a serious effort in his and others' scholarship and had learned to read in his adulthood, although he never quite learned how to write. This was quite an achievement for kings at this time, of whom most were illiterate.
Might explain Henrys interest in him?