Mishnah Negaim 13:6: A house that extends over a house with a Nega, and similarly a tree that extends over a house with a Nega, one who enters the outer one is pure, says Rabbi Elazar ben Azariah. Rabbi Elazar said, if one of its stones causes one to become impure by entering, should not [the whole house] itself make one impure by entering?!

בַּיִת שֶׁהוּא מֵסֵךְ עַל גַּבֵּי בַיִת הַמְנֻגָּע, וְכֵן אִילָן שֶׁהוּא מֵסֵךְ עַל גַּבֵּי בַיִת הַמְנֻגָּע, הַנִּכְנָס לַחִיצוֹן, טָהוֹר, דִּבְרֵי רַבִּי אֶלְעָזָר בֶּן עֲזַרְיָה. אָמַר רַבִּי אֶלְעָזָר, מָה אִם אֶבֶן אַחַת מִמֶּנּוּ מְטַמָּא בְּבִיאָה, הוּא עַצְמוֹ לֹא יְטַמֵּא בְּבִיאָה:

Mishnah Negaim 13:7: If an impure person is standing under a tree and a pure person passes [under that tree], he becomes impure. If a pure person is standing under a tree and an impure person passes [under the tree], he remains pure, but if he [the impure person] stood [under the tree], he would become impure. Similarly in the case of a stone with a Nega, [if it passes under] he remains pure, but if he laid it down, he would become impure.

הַטָּמֵא עוֹמֵד תַּחַת הָאִילָן וְהַטָּהוֹר עוֹבֵר, טָמֵא. הַטָּהוֹר עוֹמֵד תַּחַת הָאִילָן וְהַטָּמֵא עוֹבֵר, טָהוֹר. אִם עָמַד, טָמֵא. וְכֵן בְּאֶבֶן הַמְנֻגַּעַת, טָהוֹר. וְאִם הִנִּיחָהּ, הֲרֵי זֶה טָמֵא: