Thus in Holy Writ no confusion results, for all the senses are founded on one — the literal — from which alone can any argument be drawn, and not from those intended in allegory, as Augustine says (Epis. 48). Nevertheless, nothing of Holy Scripture perishes on account of this, since nothing necessary to faith is contained under the spiritual sense which is not elsewhere put forward by the Scripture in its literal sense.- Aquinas, Summa Theologica, First Part, Question 1, Article 10
I suspect that Aquinas' reference is to:
For what else is it than superlative impudence for one to interpret in his own favour any allegorical statements, unless he has also plain testimonies, by the light of which the obscure meaning of the former may be made manifest.- Augustine, Letter 93, Chapter 8, Section 24 (This letter is numbered 48 in some of the older collections, for example, this one)
That's a slightly less strong wording than Aquinas uses. In any event, there are two interesting things that Aquinas says: (1) it is improper to argue from an alleged spiritual sense, rather than from the literal sense; and (2) everything necessary for salvation cannot only be found in Scripture but it can be found in the relatively clear, literal parts (not simply in the less clear allegorical parts).
[FN1] There is reason to think that some of the analysis in the Primacy post may be mistaken. For now, we'll leave it at simply apparent, until we have more time to review the evidence behind the objections.