Concistoro Segreto del Santo Padre Paolo VI per la Nomina di Venti Cardinali
"It is in the name of Tradition that we ask all our children, all Catholic communities, to celebrate, in dignity and with fervor, the renewed Liturgy. The adoption of the new "Ordo Missae" is certainly not left to the will of the priests or of the faithful: and the Instruction of 14 June, 1971 provided for the celebration of Mass in the ancient form, with the authorization of the ordinary, only for elderly or infirm priests who offer the Divine Sacrifice sine populo.
"The Novus Ordo was promulgated to replace the old, after mature deliberation, following the requests of the Second Vatican Council. Similarly, our holy predecessor Pius V had made mandatory the Missal reformed under his authority, following the Council of Trent."