פירוש על עבודה זרה 13:2

Joseph is angry at R. Abba, not because he said something wrong, but because he said something overly obvious. It was not obvious that the halakhah follows R. Joshua b. Korha, because his was a minority opinion. But it should be obvious that the halakhah follows R. Judah because of a different principle. If the mishnah quotes a dispute and then later quotes one of those opinions anonymously, the halakhah follows the anonymous opinion. The idea is that when composing the Mishnah, R. Judah Hanasi hinted at his own opinion by citing the anonymous voice, with which he agreed, last. As we shall see, this is one of those cases.