Tosefta for Eduyot 8:3
הֵעִיד רַבִּי יְהוֹשֻׁעַ וְרַבִּי יְהוּדָה בֶן בְּתֵירָא עַל אַלְמָנַת עִסָּה, שֶׁהִיא כְשֵׁרָה לַכְּהֻנָּה, שֶׁהָעִסָּה כְשֵׁרָה לְטַמֵּא וּלְטַהֵר, לְרַחֵק וּלְקָרֵב. אָמַר רַבָּן שִׁמְעוֹן בֶּן גַּמְלִיאֵל, קִבַּלְנוּ עֵדוּתְכֶם, אֲבָל מַה נַּעֲשֶׂה, שֶׁגָּזַר רַבִּי יוֹחָנָן בֶּן זַכַּאי שֶׁלֹּא לְהוֹשִׁיב בָּתֵּי דִינִין עַל כָּךְ. הַכֹּהֲנִים שׁוֹמְעִים לָכֶם לְרַחֵק, אֲבָל לֹא לְקָרֵב:
R. Yehoshua and R. Yehudah b. Betheira testified about an issah ("dough") widow that she is permitted to (marry) a Cohein. [In a family into which a safek (possible) chalal (one unfit for the priesthood) has become intermixed, each one of that family is suspect of being that chalal. And if a woman married one of that family and her husband died, she is called an "issah" widow. For just as dough is kneaded and intermixed, so this widow is "mixed" with (i.e., compounded of) doubts: one, her husband might not be that safek chalal; and,(the other), even if he is, perhaps he is not a chalal in reality. R. Yehoshua permits her to the priesthood, for we have a "a doubt of a doubt," in which instance we rule leniently.] And R. Gamliel holds that even though generally we rule leniently in "a doubt of a doubt," here it is different, for "a special eminence is accorded to family lineage."] (And they testified) that an issah (family) is kosher both for (declaring) tamei or declaring tahor, for distancing or for drawing near. [That is, a family in which a safek chalal has become intermixed is in a status of kashruth as are all of the pedigreed families. And just as the other families may say: "This one is tamei," and they distance themselves from her, and: "This one is tahor," and they draw her near, so, (about) this family in which a safek chalal has become intermixed, we cannot say that since a safek has become intermixed in it they need no longer examine, when they marry women, which is tamei and which is tahor in order to distance her who is tamei and to draw near her who is tahor.] R. Gamliel said: We have accepted your testimony, but what can we do? For R. Yochanan b. Zakkai has decreed not to convene batei-din for this [i.e., for permitting an issah widow ab initio. For] the Cohanim will heed you to distance her [if you rule her forbidden], but not to draw her near [if you rule her permitted. The halachah is that an issah widow is forbidden to marry a Cohein ab initio, but if she does, she is not divorced.]