In his Discourse on the Forgery of the Alleged Donation of Constantine Lorenzo Valla claims that…

“[St] Gregory [the Great] in many letters testifies, the Roman ruler differs from the others, that he alone is ruler of a free people.”

“in multis epistolis Gregorius testatur, differt Romanus princeps a ceteris, quod solus est princeps liberi populi. ”

I would dearly like to find the references but nothing has turned up so far. Does anyone know where it is to be found?