The Gemara discusses a case where a man had relations with his wife and she became a nidda during the act. Abaye and Rava each quote different rabbis stating that in such a case, the man could incur an obligation of two sacrifices. Rava then explains the specific circumstances that would warrant this double punishment. The man is a Torah scholar who engages in relations with his wife when she is about to menstruate. When she informs him in the middle of the act that she has begun menstruating, he withdraws immediately. He is considered shogeg (unintentional transgressor) regarding entering the woman's body, as he incorrectly assumed he would be able to complete relations before she began menstruating. He is also considered shogeg regarding his immediate withdrawal from her body, as he, despite being a Torah scholar, was unaware of the halakha requiring him to wait until he is no longer erect before withdrawing.
Rava explains that the obligation to bring a sacrifice for each of these acts can be found in tannaitic sources. The rule about withdrawing appears in our Mishna, while the rule about entering appears in a Mishna in Nidda 14a. Rav Ada bar Matna debates with Rava whether the Mishna in Nidda actually refers to the case under discussion. Rav Ada suggests that it instead refers to withdrawal.
Rava and Abaye disagree about why a man who withdraws while not erect is exempt from bringing a sacrifice. Rava maintains that intercourse without an erection is not considered a true act of intercourse. Abaye, however, argues that the exemption exists because a situation where his wife begins menstruating during intercourse is considered beyond the person's control (ones).
Where can one find in the Torah a source for both a negative commandment (prohibition) and a positive commandment regarding a man's obligation to withdraw when not erect and to not withdraw when erect in the situation described above? Additionally, what is the source for the rabbinic prohibition against engaging in relations when a woman expects to begin her menstrual period soon?
Rabbi Eliezer and Rabbi Akiva disagree in the Mishna, though the precise point of their disagreement is unclear. Chizkia clarifies the subject of their debate.