Commentary for Kiddushin 66:14

סבר לה כר' אלעזר דאמר ר' אלעזר אין ת"ח רשאי לעמוד מפני רבו בשעה שעוסק בתורה לייט עלה אביי

or not? - Said Abaye: Come and hear: If the unclean person sits under a tree and the clean person stands, he is defiled; if the unclean person stands under the tree and the clean person sits, he remains clean; but if the unclean person sat down, the clean one is defiled. And the same applies to a leprous stone.<span class="x" onmousemove="('comment',' The reference is to a leper, who defiles a clean person when both are under the same covering overhead, but only if the leper is sitting. The boughs of a tree form such a covering. The same applies to a leprous stone. (Stones too could be leprous; v. Lev. XIV, 33-48.) If a man, bearing a leprous stone, sits under a tree, he defiles a clean man standing there; but if he stands with the stone, the other remains clean.');">¹⁶</span>

Daf Shevui to Kiddushin