Reference for Tahorot 4:3
הַשֶּׁרֶץ בְּפִי הַחֻלְדָּה, וְהַנְּבֵלָה בְּפִי הַכֶּלֶב, וְעָבְרוּ בֵין הַטְּהוֹרִים, אוֹ שֶׁעָבְרוּ טְהוֹרִים בֵּינֵיהֶן, סְפֵקָן טָהוֹר, מִפְּנֵי שֶׁאֵין לַטֻּמְאָה מָקוֹם. הָיוּ מְנַקְּרִין בָּהֶן עַל הָאָרֶץ וְאָמַר, הָלַכְתִּי לַמָּקוֹם הַלָּז וְאֵינִי יוֹדֵעַ אִם נָגַעְתִּי אִם לֹא נָגַעְתִּי, סְפֵקוֹ טָמֵא, מִפְּנֵי שֶׁיֵּשׁ לַטֻּמְאָה מָקוֹם:
If there is a vermin in the mouth of a weasel or an animal carcass in the mouth of a dog, and they passed among pure things, or pure things passed among them, their uncertainty is pure [i.e. their purity status is uncertain, and therefore pure], because the impurity has no [fixed] place. If they [the dog or weasel] were picking at them [the dead animals] on the ground, and someone says, "I went to that place, but I do not know if I touched [the vermin or the carcass] or I did not touch it, his uncertainly is impure [i.e. his purity status is uncertain, and therefore impure], because the impurity has a [fixed] place.