Mishnayos Niddah Perek 8 Mishnah 3

There was an incident involving one woman who came before Rabbi Akiva. She said to him: I saw a blood stain. Rabbi Akiva said to her: Perhaps there was a wound on your body? She said to him: Yes, there was a wound and it healed. He said to her: Was it perhaps a wound that could reopen and bleed? She said to him: Yes it was. And Rabbi Akiva deemed her ritually pure. Rabbi Akiva saw his students looking at each other, wondering why he ruled leniently in this case. Rabbi Akiva said to them: What in this matter is difficult in your eyes? The reason I ruled this way is that the Sages did not state the matter of the impurity of blood stains in order to be stringent; rather, they instituted this impurity in order to be lenient, as it is stated: “And if a woman has an issue, and her issue in her flesh shall be blood” (Leviticus 15:19), from which it is derived that by Torah law, “blood” deems her impure, but not a stain. Impurity from a blood stain was instituted by the Sages, and they rule leniently in any case where the stain can be attributed to another source.

מַעֲשֶׂה בְאִשָּׁה אַחַת שֶׁבָּאת לִפְנֵי רַבִּי עֲקִיבָא, אָמְרָה לוֹ, רָאִיתִי כָתֶם. אָמַר לָהּ, שֶׁמָּא מַכָּה הָיְתָה בִיךְ. אָמְרָה לוֹ, הֵן, וְחָיְתָה. אָמַר לָהּ, שֶׁמָּא יְכוֹלָה לְהִגָּלַע וּלְהוֹצִיא דָם. אָמְרָה לוֹ, הֵן. וְטִהֲרָהּ רַבִּי עֲקִיבָא. רָאָה תַלְמִידָיו מִסְתַּכְּלִין זֶה בָזֶה. אָמַר לָהֶם, מַה הַדָּבָר קָשֶׁה בְעֵינֵיכֶם. שֶׁלֹּא אָמְרוּ חֲכָמִים הַדָּבָר לְהַחְמִיר אֶלָּא לְהָקֵל, שֶׁנֶּאֱמַר (ויקרא טו), וְאִשָּׁה כִּי תִהְיֶה זָבָה דָּם יִהְיֶה זֹבָהּ בִּבְשָׂרָהּ, דָּם וְלֹא כָתֶם: