Tractate Oholot - Chapter 10 - Mishnah 2 אהלות פרק י׳ משנה ב׳

If the hatchway does not have an opening of a square handbreadth: If there is uncleanness in the house, what is directly [below] the hatchway remains clean. If the uncleanness is directly [below] the hatchway, the house remains clean. When the uncleanness is in the house, if he placed his leg above [the hatchway], he remains clean. [When] the uncleanness is directly [below] the hatchway, if he placed his leg above it, Rabbi Meir declares [him] unclean, But the sages say: if the uncleanness was [in position] before his leg, he becomes unclean, but if his leg was [in position] before the uncleanness, he remains clean. Rabbi Shimon says: [in the case where] two [men's] legs, one above the other, were [in position] before the uncleanness, if the first person withdrew his leg and the other person's leg was still there, [the second] remains clean, because the first person's leg was [in position] before the uncleanness.