De institutione musica II.19: De ordine consonantiarum sententia Eubulidis et Hippasi.
But Eubulides and Hippasus set down another order of the consonances. For they say that the increases of the multiple correspond, in a fixed order, to the decrease of the superparticular. Thus there can be no double without a half, nor a triple without a third part. Since therefore there is a double, from it the consonance of the octave is produced; but since there is a half, from it, as by a contrary division, the sesquialter proportion, that is, the fifth, is brought about. When these are mixed, namely the octave and the fifth, the triple is begotten, which contains both symphonies. But again, the triple is divided by a contrary division into its third part, from which in turn the symphony of the fourth will be born. The triple and the sesquitertian joined make the quadruple comparison of proportion. Hence it comes about that from the octave and fifth, which is one consonance, and the fourth, one concord, there is joined that which, consisting in the quadruple, has received the name double octave. According to these men also the order is this: octave, fifth, octave and fifth, fourth, double octave.