Origins of "vox populi, vox Dei"

In a letter from Alcuin to Charlemagne from 798 we have the earliest use of "vox populi, vox Dei". Note, the use is not positive:

Nec audiendi qui solent dicere, Vox populi, vox Dei, quum tumultuositas vulgi semper insaniae proxima sit.

English translation:

And those people should not be listening to those who keep saying the voice of the people is the voice of God, since the riotousness of the crowd is always very close to madness.