פירוש על קידושין 141:7

R. Yohanan swears by the Temple that the rabbis can reveal who has poor lineage. The problem is that some of the greatest men of their time are already mixed up with them and do not want to have their genealogical flaws revealed. R. Yohanan agrees with R. Yitzchak that once a family has their lineage mixed up, there is nothing anyone should do about it.