Furthermore, I send you, as a blessing from Saint Peter the apostle, a key from his most sacred body; with respect to which key the miracle has been wrought which I now relate. A certain Lombard, having found it on his entrance into a city in the parts beyond the Po, and, paying no regard to it as Saint Peter's key, but wishing to make something of it for himself in that he saw it to be of gold, took out a knife to cut it. But presently seized by a spirit, he plunged the knife wherewith he had thought to cut it into his own throat, and in the same hour fell down dead.
And when Autharith, king of the Lombards, and many others belonging to him came to the place, and he who had stabbed himself was lying apart in one place dead, and this key on the ground in another, exceeding fear came upon all, so that no one ventured to lift this same key from the ground. Then a certain Lombard who was a Catholic, and known to be given to prayer and almsgiving, Minulf by name, was called, and himself lifted it from the ground. But Autharith, in consideration of this miracle, made another golden key, and sent it along with this to my predecessor of holy memory, declaring what kind of miracle had through it occurred.
I have taken thought, then, to send your Excellence this key, through which Almighty God cut off a proud and faithless man, that through it you who fear and love Him may be enabled to have both present and eternal welfare.
Source: Register of Letters (New Advent)