Hmm. Good question, it's everywhere so I didn't question it particularly, since it also fit with other statements.
Google says: (Quaest. in IV Sent.; quoted in Viollet, Papal Infallibility and the Syllabus, 1908)
Whatever Quaest in IV Sent; is shorthand for, I do not know, nor its level of authority. There's a claim from some fellow on the net, that the statement was made before the Pope was Pope and was secondhand from his students.
I was able to find the secondary source cited as:
"during the reign of St. Pius X, this work was placed on the Index of Forbidden Books. (Decree, 5 April 1906. See R. Naz, “Viollet, Paul-Marie,” Dict. Droit. Can., 7:1511)
I suppose the quote can be challenged if the background is what is claimed.
OTOH it does fit with other statements such as:
"If a future pope teaches anything contrary to the Catholic Faith, do not follow him."
Bl. Pope Pius IX (Letter to Bishop Brizen)
After his death, Honorius was anathemized by the Eastern Church; but we should not forget that he was accused of heresy, the only crime that would make lawful the resistance of inferiors to the orders of their superiors, and the refusal of their malicious doctrines.
Pope Adrian II
“The pope should not flatter himself about his power, nor should he rashly glory in his honour and high estate, because the less he is judged by man, the more he is judged by God. Still the less can the Roman Pontiff glory, because he can be judged by men, or rather, can be shown to be already judged, if for example he should wither away into heresy, because “he who does not believe is already judged.” (St. John 3:18) In such a case it should be said of him: ‘If salt should lose its savour, it is good for nothing but to be cast out and trampled under foot by men.’”
Pope Innocent III
Etc. I'm glad I looked up further on the quote's sourcing, but my real focus was and is on who are those referred to in it. How dark is history on this? I get the idea that nobody knows in general.