Tosefta for Tahorot 2:8
הָאוֹכֵל אֹכֶל שֵׁנִי, לֹא יַעֲשֶׂה בְּבֵית הַבָּד. וְחֻלִּין שֶׁנַּעֲשׂוּ עַל גַּב קֹדֶשׁ, הֲרֵי אֵלּוּ כְחֻלִּין. רַבִּי אֶלְעָזָר בַּר רַבִּי צָדוֹק אוֹמֵר, הֲרֵי אֵלּוּ כִתְרוּמָה, לְטַמֵּא שְׁנַיִם וְלִפְסֹל אֶחָד:
One who is eating foods of a second degree [impurity] should not work in an olive press [since his body becomes of a second degree, and he then renders liquids impure]. And non-sacred foods that were prepared with the stringency of sacred foods still follow the rules of non-sacred foods [i.e. unlike sacred foods, they cannot be rendered third degree by a second degree impurity, nor are they rendered fourth degree by a third degree impurity]. Rabbi Elazar bar Rabbi Tzadok says: In fact they are like <i>terumah</i>, in that [if they touch an origin of impurity] two are rendered impure [those rendered of a first and second degree impurity] and one is invalidated [that of a third degree; and it does not produce a fourth].