Tractate Keritot - Chapter 1 - Mishnah 6 כריתות פרק א׳ משנה ו׳

If a woman miscarries on the eve of the eighty-first day: Bet Shammai say: she is exempted from an offering. But Bet Hillel say: she is liable. Bet Hillel said to Bet Shammai: what is the difference between the eve of the eighty-first day and the eighty-first day itself? Since these are considered equal with regard to [blood] uncleanness, why should they not be considered equal also with reference to the offerings? Bet Shammai said to them: No; if you said this in the case where she miscarries on the eighty-first day where it occurred at a time when she was fit to bring an offering, can you say this where she miscarries on the eve of the eighty-first day, where it did not occur at a time when she was fit to bring an offering? Bet Hillel said to them: the case of a woman who miscarries on the eighty-first day which fell on a Shabbat shall prove it, where it did not take place at a time when she was fit to bring an offering and yet she is liable to bring a [new] offering. Bet Shammai said to them: No; if you says this of the eighty-first day which fell on a Shabbat for although it is not fit for offerings of an individual, it is at least fit for communal offerings, would you maintain this concerning a woman who miscarries on the eve of the eighty-first day, seeing that the night is fit neither for offerings of the individual nor for communal offerings? As to [your argument of the uncleanness of] the blood, it proves nothing, for if she aborted within the period of cleanness the blood is unclean, and yet she is exempted from an offering.