Niddah 9:2-3

Niddah 9:2

If a man and a woman both emptied their bladders into the same chamber pot and blood was found in their urine, Rabbi Yosi rules the woman clean. Rabbi Shimon rules her unclean because men do not typically discharge blood, so the blood is presumed to have come from the woman.

Niddah 9:3

If a woman lent her shirt to a non-Jewish friend or to a niddah, she may attribute a bloodstain to the borrower. If three women wore the same shirt or sat on the same bench and blood is found on it, all of them are ritually unclean. If they sat on a stone bench or on a bathhouse bench (which is fixed to the ground), Rabbi Nechemya rules them clean on the basis that anything insusceptible to impurity is also insusceptible to bloodstains.