The Catholic Encyclopedia

The authorship is now generally ascribed to Hermann Contractus… Durandus, in his "Rationale", ascribed it to Petrus of Monsoro (d. about 1000), Bishop of Compostella. It has also been attributed to Adhémar, Bishop of Podium (Puy-en-Velay), whence it has been styled "Antiphona de Podio" (Anthem of Le Puy).