Tosefta על עדיות 8:3

A male convert and freed slave are permitted [to marry] a mamzeret and the child is a mamzer—words of Rabbi Yoseh. Rabbi Yehudah says: A male convert can't marry a female convert. A male convert, freed slave and disqualified priest are permitted [to marry] a female priest. Which is [a person whose lineage is mixed like] dough (isah, see Mishnah Eduyot 8:3)? Anyone [with mixed lineage but] who doesn't have [any suspicion of having ancestors with] netinut, mamzerut or kings' slaves. Said Rabbi Meir: Anyone who doesn't have netinut, mamzerut or kings' slaves [in their heritage]—they can marry [the person] into the priesthood. Rabbi Shimon ben Lazar says in the name of Rabbi Meir, and so too Rabbi Shimon ben Menasya used to say the same: Why did they say that an isah is unfit [to marry] into the priesthood? Because of the possibility of disqualified priests [in his lineage] that they would become impure through—Jews know the netinim and mamzerim among them, but they don't know the disqualified priests among them.