De institutione musica III.6: Tonum ex duobus semitoniis et commate constare.
From these it is easily apparent that the tone consists of two lesser semitones and a comma. For if the whole tone consists of an apotome and a semitone, and the semitone differs from the apotome by a comma, the apotome is nothing other than the lesser semitone and a comma. If, then, anyone takes two lesser semitones from a tone, the comma is what remains.