Legend claims the St. Gregory the Great (d. 604) listened to the very first 3 lines chanted by angels on a particular Easter early morning in Rome although he walked barefoot in a great religious procession and which the saint thereupon added the fourth line: “Ora Professional nobis Deum. Alleluia.” https://vitapronobis32357.bloggazzo.com/30510946/vita-pro-nobis-an-overview