De institutione musica I.7: Quae proportiones quibus consonantiis musicis aptentur.
This, however, must be known: that all musical consonances consist either in the double or the triple or the quadruple or the sesquialter or the sesquitertian proportion; and the proportion that in numbers is sesquitertian is called in sounds the diatessaron; that which in numbers is sesquialter is called in voices the diapente; that which in proportions is double is the diapason among consonances; the triple is diapente and diapason; the quadruple is the double diapason. And for now this is said universally and without distinction; later the whole account of the proportions will be made clear.