This can be found in book 3, chapter 1 of the Extravagantes communes of pope John, entitled, De Vita et honestate clericorum. A complete English translation of this short section can be found in Robert Hayburn's, Papal Legislation on Sacred Music. The Bull was actually not so much about the primacy of chant, but about eliminating the abuses that had set in when the liturgy was sung to new contrapuntal compositions, such as use of the vernacular, dissonances, too hidden plainchant melodies and their rhythmic modifications, and the general difficulty of following the words.