Although this is forbidden, such relations are not termed "Zenut": Temurah 29b
Terming such relationships "Toevah": Temurah 29b
The penalty of kareit - Divine Excommunication: Keritot 2a
The penalty of bringing a korban chatat [sin offering]: Keritot 2a
Bringing a korban asham taluy [offering for uncertain guilt] if blood was found, after relations, on a cloth with which he wiped himself, or she wiped herself: Keritot 17b-18a
The penalty for relations with multiple niddot during a single span, or multiple spans, of forgetting the prohibition: Keritot 15a, 16a
The penalty for relations with a niddah during multiple periods of niddah status: Keritot 17a
Her liability for having relations with a man during a niddah period: Keritot 17a
Impurity of a man who slept with a woman shortly before she discovered blood: Niddah 6a, 14a
Impurity of a man who slept with a woman shortly before she discovered a bloodstain: Niddah 6a, 14a, 15a
Impurity of a man who slept with a woman shortly after she discovered a bloodstain: Niddah 14a, 15a
Status of the man while he is impure
Whether he may learn Torah before immersion: Berachot 22a; Moed Katan 15a
