Mishnah Tevul Yom 4:2: A woman who is a tevulat yom may knead dough, cut off a portion for challah, and separate it, and place it in a basket of twigs or on a tray, and then bring it near [to the rest of the dough] and call it a name [i.e. declare it to be challah]; because it is of a third degree [impurity], and something of a third degree is pure with regard to chullin.

הָאִשָּׁה שֶׁהִיא טְבוּלַת יוֹם, לָשָׁה אֶת הָעִסָּה, וְקוֹצָה לָהּ חַלָּה וּמַפְרַשְׁתָּהּ, וּמַנַּחְתָּהּ בִּכְפִישָׁה מִצְרִית אוֹ בִנְחוּתָא, וּמַקֶּפֶת וְקוֹרָא לָהּ שֵׁם, מִפְּנֵי שֶׁהִיא שְׁלִישִׁי, וְהַשְּׁלִישִׁי טָהוֹר לְחֻלִּין:

Mishnah Tevul Yom 4:3: Regarding a [kneading] trough which is a tevulat yom [i.e. which was immersed that day], one may knead dough in it, and cut off a portion for challah, and bring [the rest of the dough] near, and call it a name [of challah]; because it is of a third degree [impurity], and something of a third degree is pure with regard to chullin.

עֲרֵבָה שֶׁהִיא טְבוּלַת יוֹם, לָשִׁין בָּהּ אֶת הָעִסָּה וְקוֹצִין מִמֶּנָּה חַלָּה, וּמַקֶּפֶת וְקוֹרִין לָהּ שֵׁם, מִפְּנֵי שֶׁהִיא שְׁלִישִׁי, וְהַשְּׁלִישִׁי טָהוֹר לְחֻלִּין: