Gregory I, Pope. [Letter to Theodelind, queen of the Lombards, 593] in Gregorii I papae Registrum epistolarum 4.4, 1.236

[Letter from Pope Gregory I to Theodelind, queen of the Lombards]
The pope warns the queen not to believe bishops who say the doctrine established by the council of Chalcedon has been modified.
Online at Epistolae (See also Bibliographia record).