In Summa Theologica I q. 1 a. 10 ("Whether in Holy Scripture a word may have several senses?") ad 1, Aquinas writes:
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.
What writing of Augustine is referred to by [Epis. 48] here?
There is a Letter 48 from Augustine, but the content seems disconnected from the reference here.