The Church has never accepted even the most holy and most eminent Doctors, and does not now accept even a single one of them, as the principal source of truth. The Church certainly considers Thomas and Augustine great Doctors, and she accords them the highest praise; but, by divine mandate, the interpreter and guardian of the Sacred Scriptures and depository of Sacred Tradition living within her, the Church alone is the entrance to salvation; she alone, by herself, and under the protection and guidance of the Holy Ghost, is the source of truth.
I found this one. Here is exactly how it appears on page 317:
The Church has never accepted, and does not now accept as the principal source of truth, even the most holy and most eminent Doctors. Certainly, the Church considers Thomas and Augustine great Doctors, and she accords them the highest praise; but she recognizes infallibility only in the inspired authors of Sacred Scripture. Being by divine mandate the interpreter and guardian of the Sacred Scriptures, the Church alone is the entrance to salvation, and depository of the Sacred Tradition living within her. She alone, by herself, under the protection and guidance of the Holy Spirit, is the source of truth. Pope Pius XII
The reference for the quote given in the index (p. 546) is as follows: Pius XII ALLOCUTION: to the Gregorian, October 17, 1953, PTC 1351.
In the “Key to Bibliography” section (third from the bottom on page 477), the abbreviation PTC is explained as follows: PAPAL TEACHINGS: THE CHURCH, selected and arranged by the Benedictine monks of Solesmes, Boston: St. Paul Editions, 1962 (paragraph numbers cited)