The following most likely comes from the Sermons of St. Leo the Great, although it is rather difficult to be certain given the site I was using. It comes from Series II Volume XII of the Church Fathers.
from "Sermon XLII. On Lent, IV"
St. Leo the Great
IV. The Perverse Turn Even Their Fasting into Sin.
This adversary's wiles then let us beware of, not only in the enticements of the palate, but also in our purpose of abstinence. For he who knew how to bring death upon mankind by means of food, knows also how to harm us through our very fasting, and using the Manichaeans as his tools, as he once drove men to take what was forbidden, so in the opposite direction he prompts them to avoid what is allowed. It is indeed a helpful observance, which accustoms one to scanty diet, and checks the appetite for dainties: but woe to the dogmatizing of those whose very fasting is turned to sin. For they condemn the creature's nature to the Creator's injury, and maintain that they are defiled by eating those things of which they contend the devil, not God, is the author: although absolutely nothing that exists is evil, nor is anything in nature included in the actually bad. For the good Creator made all things good and the Maker of the universe is one, "Who made the heaven and the earth, the sea and all that is in them." Of which whatever is granted to man for food and drink,' is holy and clean after its kind. But if it is taken with immoderate greed, it is the excess that disgraces the eaters and drinkers, not the nature of the food or drink that defiles them. "For all things," as the Apostle says, "are clean to the clean. But to the defiled and unbelieving nothing is clean, but their mind and conscience is defiled."
This is of particular interest to those who would argue the evil of material things. Don't think there's many of us about, but a few hard-line protestants and some renegade members of various Catholic camps.