De institutione musica II.26: Diapason ac diapente in tripla proportione esse, in quadrupla bis diapason.
But since it has been demonstrated that the octave is double, the fifth sesquialter, and that the double and the sesquialter joined create the triple proportion, from these things it is also plain that the fifth and the octave are established in the triple proportion. But if anyone joins the sesquitertian relation to the triple proportion, he makes the quadruple. Therefore if the symphony of the fourth is joined to the consonances of the fifth and the octave, the quadruple space of voices arises, which we have shown above to be the double octave.