De institutione musica II.25: Diapente in sesqualtera, diatessaron in sesquitertia esse, tonum in sesquioctava.
...the tone in the sesquioctave. XXV. I say, however, that the fifth properly consists in the sesquialter, the fourth in the sesquitertian proportion.
For since between the two proportions, namely the sesquialter and the sesquitertian, the sesquialter is the greater and the sesquitertian the lesser, and since among the consonances the fifth is the greater and the fourth the lesser, it is plain that the greater proportion is to be fitted to the greater consonance, the lesser to the lesser. The fifth, therefore, will be placed in the sesquialter, the fourth in the sesquitertian proportion. If we subtract the fourth from the consonance of the fifth, there remains the space which is called a tone. And if we take the sesquitertian from the sesquialter proportion, the sesquioctave proportion remains.
Hence it comes about that the tone ought to be established in the sesquioctave comparison.