John Cassian
Tradition says that Benedict of Nursia introduced this custom into the monastic Office, heavily influenced by the writings of Saint John Cassian. Saint Gregory the Great extended it to all the Roman churches. Saint John Cassian (Coll., X, 10) wrote that from the earliest Christian times the monks used this introduction very often, including outside of the liturgical prayers, as an invocation for every situation, for times of temptation, tiredness, and joy.