Reference for Eduyot 4:12
אָדָם שֶׁהוּא נָתוּן תַּחַת הַסֶּדֶק, בֵּית שַׁמַּאי אוֹמְרִים, אֵינוֹ מֵבִיא אֶת הַטֻּמְאָה. וּבֵית הִלֵּל אוֹמְרִים, אָדָם חָלוּל הוּא, וְהַצַּד הָעֶלְיוֹן מֵבִיא אֶת הַטֻּמְאָה:
[In a vestibule whose roof split in two, where there are vessels (on the ground) on one side of the split and an unclean object (tumah) on the other side,] if a man were lying (on the ground) [directly] under the split — Beth Shammai say: [The vessels are clean, for] the man does not conduct tumah [from the object to the vessels, only something having a cavity the size of a tefach doing so, (and the air under the split acting as a "barrier" between the object and the vessels)]. And Beth Hillel say: A man is [considered as having] a cavity [And even though his intestines are within it, the cavity within the body is regarded as a cavity of a tefach], and the upper part (of his body) conducts the tumah (to the vessels).