Mishnayos Niddah Perek 9 Mishnah 4

In a case of three women who were sleeping in one bed and blood was discovered beneath one of them, all of them are ritually impure. If when the blood was discovered one of them examined herself and discovered that she was impure due to menstruation, she is impure and the other two are pure. And if none of them examined themselves, or if all of them examined themselves and were pure, they attribute the blood to each other, i.e., if one of them is unfit to menstruate, e.g., she is pregnant, she may attribute the blood to the other women who are fit to menstruate. And if all three women were not fit to see the flow of blood, e.g., they each belonged to one of the categories listed in the mishna on 7a, one considers them as though they were fit, and all three are impure, because the blood must have originated from one of them.

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: