Legend suggests the St. Gregory the Great (d. 604) listened to the very first a few lines chanted by angels on a particular Easter early morning in Rome although he walked barefoot in a great religious procession and the saint thereupon added the fourth line: “Ora pro nobis Deum. Alleluia.” https://vita-pro-nobis42187.link4blogs.com/52873145/not-known-facts-about-vita-pro-nobis