I’ve used Google books to research the origin of a well-known, but often rephrased quote by Pope John XXIII on papal infallibility. Here is what I’ve found:

In an audience before a group of Greek seminarians, Pope John XXIII said:
“I am not infallible.”
When that statement had had the desired effect, he explained:
“The pope is only infallible when he speaks ex cathedra.
I will never speak ex cathedra, therefore I am not infallible.”

Sources:

“Listening” magazine, Volumes 1-2 (1966), page 181;
Aquinas Institute of Philosophy and Theology.

“Commonweal” magazine, Volume 102 (1975), page 144
Commonweal Publishing Corporation.