Legend states the St. Gregory the Great (d. 604) listened to the very first a few lines chanted by angels on a specific Easter morning in Rome when he walked barefoot in a fantastic spiritual procession and that the saint thereupon included the fourth line: “Ora pro nobis Deum. Alleluia.” https://dores-nos-nervos99764.getblogs.net/63700382/the-5-second-trick-for-vita-pro-nobis