Mishnah Niddah

m. Niddah 2:4

All women are presumed to be ritually pure for [the purpose of sexual intercourse with] their husbands; those that return home from a journey—their wives are presumed to be ritually pure for [the purpose of sexual intercourse with] them.

m. Niddah 8:2–3

2. [A woman] may attribute [a bloodstain] to any [external] cause to which she can possibly attribute it. If [for instance] she had slaughtered a beast, a wild animal, or a bird. Or if she was handling bloodstains, or if she sat beside those who handled them. Or if she killed a louse, she may attribute the bloodstain to it. How large a stain may be attributed to a louse? R. Ḥanina ben Antigonus says: One up to the size of a split bean. And even if she did not kill it. She may also attribute it to her son or to her husband. If she herself had a wound that could open again and bleed, she may attribute it to it.

3. It happened that a woman came before R. Akiva and said to him, “I have seen a bloodstain.” He said to her, “Perhaps you had a wound?” She said to him, “Yes, but it has healed.” He said to her, “Perhaps it could have opened again and let out some blood.” She said to him, “Yes.” And R. Akiva declared her pure. He saw his disciples look at each other in astonishment. He said to them: Why do you find this difficult? For the sages did not say this rule in order to be stringent but rather to be lenient, as is written: And if a woman have issue, and her issue in her flesh be blood (Leviticus 15:19)—blood but not a bloodstain.