What work of Augustine is Aquinas Citing Here? 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.
The English translation of that passage is incomplete. The Latin of Summa Theologica I q. 1 a. 10 says

…ut dicit Augustinus in epistola contra Vincentium Donatistam.
…as St. Augustine says in his letter against the Donatist Vincent.

Opera omnia, iussu impensaque Leonis XIII (vol. 4): Summa Iᵃ q. 1-49 (p. 25) gives this as the full reference:

Ad Vincent. Rogat. Ep. XCIII (al. XLVIII), c. VIII.

Thus: chapter 8 of letter 93 (45 in another numbering scheme).
Relevant passage:

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.