Tractate Niddah - Chapter 8 - Mishnah 3 נידה פרק ח׳ משנה ג׳

It happened that a woman came in front of Rabbi Akiva and said. She 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 Rabbi Akiva declared her clean. He saw his disciples looked 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, for it is said, "And if a woman have issue, and her issue in her flesh be blood" blood but not a bloodstain.