In telling how Pope Leo persuaded Attila not to attack the city of Rome, Gibbon is more sympathetic to later legendary accretions to the story than he usually is: Gibbon Chapter 35
"The apparition of the two apostles of St. Peter and St. Paul, who menaced the barbarian with instant death if he rejected the prayer of their successor, is one of the noblest legends of ecclesiastical tradition. The safety of Rome might deserve the interposition of celestial beings; and some indulgence is due to a fable which has been represented by the pencil of Raphael and the chisel of Algardi."
I have added links to images of the artworks Gibbon is referring to.
The Internet Medieval Sourcebook gives two accounts which illustrate the development of the story of the meeting between Leo and Attila. In a footnote, Bury suggests that the story of the two saints accompanying the Pope to ward off Attila from Rome may have come from an oblique reference in Leo's Sermon No. 84:
"Who was it that restored this city to safety? that rescued it from captivity? the games of the circus-goers or the care of the saints? surely it was by the saints’ prayers that the sentence of Divine displeasure was diverted, so that we who deserved wrath, were reserved for pardon."
Peter and Paul don't seem to be on the cast list for the 1950s Sophia Loren film "Attila",which I saw on TV a very long time ago, but then neither is Leo, who I distinctly remember being punted through the marshes on his way to see Attila.
