Institutio Oratoria
Quintilian
Quintilian. Institutio Oratoria, Volume 1-4. Butler, Harold Edgeworth, translator. Cambridge, Mass; London: Harvard University Press, William Heinemann Ltd., 1920-1922.
On all such subjects the teacher must use his own judgment; for in such matters it should be the supreme authority. For my own part, I think that, within the limits prescribed by usage, words should be spelt as they are pronounced.
For the use of letters is to preserve the sound of words and to deliver them to readers as a sacred trust: consequently they ought to represent the pronunciation which we are to use.