Reference for Mikvaot 4:5
הַשֹּׁקֶת שֶׁבַּסֶּלַע, אֵין מְמַלְּאִין מִמֶּנָּה, וְאֵין מְקַדְּשִׁין בָּהּ, וְאֵין מַזִּין מִמֶּנָּה, וְאֵינָהּ צְרִיכָה צָמִיד פָּתִיל, וְאֵינָהּ פּוֹסֶלֶת אֶת הַמִּקְוֶה. הָיְתָה כְלִי וְחִבְּרָהּ בְּסִיד, מְמַלְּאִין בָּהּ, וּמְקַדְּשִׁין בָּהּ, וּמַזִּין מִמֶּנָּה, וּצְרִיכָה צָמִיד פָּתִיל, וּפוֹסֶלֶת אֶת הַמִּקְוֶה. נִקְּבָה מִלְּמַטָּה אוֹ מִן הַצַּד וְאֵינָהּ יְכוֹלָה לְקַבֵּל מַיִם כָּל שֶׁהֵם, כְּשֵׁרָה. וְכַמָּה יִהְיֶה בַנֶּקֶב. כִּשְׁפוֹפֶרֶת הַנּוֹד. אָמַר רַבִּי יְהוּדָה בֶּן בְּתֵירָא, מַעֲשֶׂה בְשֹׁקֶת יֵהוּא שֶׁהָיְתָה בִירוּשָׁלַיִם, וְהָיְתָה נְקוּבָה כִשְׁפוֹפֶרֶת הַנּוֹד, וְהָיוּ כָל הַטָּהֳרוֹת שֶׁבִּירוּשָׁלַיִם נַעֲשׂוֹת עַל גַּבָּהּ, וְשָׁלְחוּ בֵית שַׁמַּאי וּפְחָתוּהָ, שֶׁבֵּית שַׁמַּאי אוֹמְרִים, עַד שֶׁיִּפְחֲתוּ רֻבָּהּ:
Regarding a trough in a rock [in which water gathers fromn a nearby spring], one may not fill [water to be sanctified with ashes of the red heifer] from it, and one may not sanctify [water with red heifer ashes] in it, and one may not sprinkle [sanctified waters, as part of the purification ritual,] from it, and it does not require a 'tight-seal' [to protect its contents from impurity, rather a mere covering suffices], and it does not invalidate a <i>mikveh</i> [if water gathered in the trough and then flowed into a <i>mikveh</i>, it is not considered drawn water]. If a vessel was attached [to the ground] with plaster, one may fill [water for sanctification] with it, and one may sanctify [water] in it, and one may sprinkle from it, and it requires a 'tight-seal' [to protect its contents from impurity], and it invalidates a <i>mikveh</i> [if water flowed in from it, it is considered to be drawn water and thus invalid]. If it was punctured from below or from the side, and it cannot hold any amount of water, it [any <i>mikveh</i> filled with its waters] is valid. And how far must the hole span? Like [the width of] the tube of a flask. Rabbi Yehuda ben Beteira says: It happened regarding the Yehu trough in Jerusalem, which was punctured like the tube of flask, and upon which all the purifications of Jerusalem were done, and Beit Shammai sent [emissaries] and diminished it, since Beit Shammai say: [even if it has a small hole, it is still considered a vessel and thus invalidates a <i>mikveh</i>] until the majority of it is diminished.