De institutione musica III.7: Demonstratio tonum duobus semitoniis commate distare.
The same will be proved also in this way. For if the octave is contained in five tones and two lesser semitones, and six tones exceed the consonance of the octave by one comma, there is no doubt that, when five tones are taken away from each space, there remain from the octave two lesser semitones, but from the six tones one tone. And this tone will exceed these two semitones that are left by a comma. If the comma is put back to the same two semitones, they will equal the tone. It is therefore established that one tone is equal to two lesser semitones and a comma, which is found in the first 7,153 units.