נדה ט׳Niddah 9

Mishnah 3ג׳

הִשְׁאִילָה חֲלוּקָהּ לְנָכְרִית אוֹ לְנִדָּה, הֲרֵי זוֹ תּוֹלָה בָהּ. שָׁלשׁ נָשִׁים שֶׁלָּבְשׁוּ חָלוּק אֶחָד אוֹ שֶׁיָּשְׁבוּ עַל סַפְסָל אֶחָד, וְנִמְצָא עָלָיו דָּם, כֻּלָּן טְמֵאוֹת. יָשְׁבוּ עַל סַפְסָל שֶׁל אֶבֶן אוֹ עַל הָאִצְטַבָּא שֶׁל מֶרְחָץ, רַבִּי נְחֶמְיָה מְטַהֵר, שֶׁהָיָה רַבִּי נְחֶמְיָה אוֹמֵר, כָּל דָּבָר שֶׁאֵינוֹ מְקַבֵּל טֻמְאָה, אֵינוֹ מְקַבֵּל כְּתָמִים:

If a woman lent her robe to a gentile woman or to a niddah [a female who has menstrual discharges which render her impure], she may attribute [any bloodstain] to her. If three women wore the same robe or sat on the same bench, and blood was then found on it, they are all impure. If they sat on a stone bench or on [a ledge in] the colonnade of a bath house, Rabbi Nechemia considers them pure, for Rabbi Nechemia would say: Anything that cannot be rendered impure cannot retain a stain.

Mishnah 4ד׳

שָׁלשׁ נָשִׁים שֶׁהָיוּ יְשֵׁנוֹת בְּמִטָּה אַחַת וְנִמְצָא דָם תַּחַת אַחַת מֵהֶן, כֻּלָּן טְמֵאוֹת. בָּדְקָה אַחַת מֵהֶן וְנִמְצֵאת טְמֵאָה, הִיא טְמֵאָה וּשְׁתֵּיהֶן טְהוֹרוֹת, וְתוֹלוֹת זוֹ בָזוֹ. וְאִם לֹא הָיוּ רְאוּיוֹת לִרְאוֹת, רוֹאִין אוֹתָן כְּאִלּוּ הֵן רְאוּיוֹת:

If three women were sleeping in one bed and blood was found under one of them, they are all impure. If one of them examined [herself] and was found to be impure, she is impure and the other two are pure; and [even if none of them examined] they may attribute to one another [if there is any reason to assume one of them is more or less fit to see blood]. And if none of them are fit to see [blood, i.e. if they were virgins or pregnant, etc.], we view all of them as though they are fit [to see blood].