Mysterious Scripture Passage

In the first "epistle of Clement" of which our single copy comes from Codex Alexandrinus, a passage which does not exist in known scripture is quoted--being referenced like unto other scripture (it is written). Some people believe--and there is reason for this belief--that this epistle may have been written by the Clement mentioned by Paul himself in Philippians 4:3. If true, that would make the authorship of this epistle very early in church history--early enough so that it would argue strongly against the passage being a concoction. The question then is...where did it come from? Is this simply a creative mixture of other portions of known scripture?

Far from us be that which is written, “Wretched are they who are of a double mind, and of a doubting heart; who say, These things we have heard even in the times of our fathers; but, behold, we have grown old, and none of them has happened unto us."
--1st Clement, 23:3