Listeners to my podcasts and readers of my books and articles have asked me to provide them with the source of a patristic principle which I have often quoted during my lectures, "τό καλόν οὐκ ἔστι καλόν, ἐάν μή καλῶς γένηται" (good is not good, if not done in a good way). Having heard this many times over my 12 years at the theological school in Thessaloniki, it had become so ingrained in me that I essentially took it for granted and had forgotten where I had originally seen it in the writings of the Holy Fathers.
After a search was done, it turns out that it is used by St Ephraim the Syrian, and multiple times by St. John of Damascus. Two of the several relevant passages are pasted below.
I am indebted to Fr. Theodore and Fr. Seraphim Zisis for their assistance.