De institutione musica I.11: Quibus modis variae a Pythagora proportiones consonantiarum perpensae sint.
Returning home from there, he weighed with various trials whether the whole reason of the symphonies consisted in these proportions. Now fitting equal weights to strings and judging their consonances by ear, now restoring the doubling and halving in the length of reeds and fitting the other proportions, he took in by varied experience a most complete confidence. Often, too, he put cups of equal weights into vessels according to the measure of their quantities; often, striking the vessels themselves, formed of different weights, with a rod of bronze or iron, he rejoiced to have found nothing different. Led from this, he set about examining the length and thickness of strings. So he found the rule, of which I shall speak later, which took its name from the thing, not because it is a wooden rule by which we measure the sizes of strings and the sound, but because such an inspection is a fixed and firm rule, so that it deceives no inquirer by a doubtful sign.