Midrash su Levitico 11:36

1) (Vayikra 11:36) ("But a spring or a pit where waters are gathered shall be clean; and he who touches their carcass [the carcass of the animals mentioned above] shall be tamei.") If it were written (only) "a gathering of waters shall be clean," I would think that even if he filled up (containers of water and carried them) on his shoulder and made a mikvah ab initio that it would be valid; it is, therefore, written "a spring" — Just as a spring is made by Heaven, so a mikveh must be made by Heaven. — But if so, I would say: Just as there is no involvement of man in a spring, so there must be no involvement of man in a mikveh — to exclude one's placing his vessels on the roof to dry, their filling up with (rain) water, (their breaking, and filling up a pit, in which instance I would think that the water is considered "drawn" and that the pit is pasul as a mikvah) — it is, therefore, (to negate this,) written "a pit." If so, I would think that a pit in a boat (i.e., a cavity into which sea water is admitted) would be valid; it is, therefore, written "a spring." Just as a spring is ground-based, a pit, too, must be ground-based. If so, I might think that just as a spring cleanses with any amount (that covers the object), so a mikvah cleanses with any amount. It is, therefore, written "But a spring" — a spring cleanses with any amount, but a mikvah requires forty sa'ah.

Another matter, “as your love [dodekha] is better than wine”; matters of Torah are similar to each other, they are companions to one another, they are close to each other, just as you say: “Or his uncle [dodo] or his cousin” (Leviticus 25:49).¹²⁶Just as the term dod indicates a familial relationship, that of an uncle (dodo, his dod, means his uncle), the term dodekha, “your dod,” in the verse, indicates a close relationship. This is interpreted as referring to matters of Torah, which are close to each other in the sense that one passage sheds light on another. “However, a spring, or cistern, a gathering of water, [shall be pure]” (Leviticus 11:36), [yet] render food susceptible to impurity, as it is stated: “But if water shall be placed on a seed […it shall be unclean for you]” (Leviticus 11:38).¹²⁷This verse teaches the law that food items become susceptible to impurity only if they have already come into contact with water or another liquid. One might have thought that a pool of water that can render an impure item pure, such as those mentioned in verse 36, would not render food items susceptible to impurity. The juxtaposition of the verses teaches that the water referred to in verse 38 includes even the types of water in verse 36, and although these pools of water can render an impure item pure, they also render food items susceptible to impurity. This is an example of how one verse can shed light on another.
Shimon bar Abba [said] in the name of Rabbi Yoḥanan: The words of the scribes¹²⁸The Oral Law transmitted by the Sages, as well as rabbinic ordinancs. are as dear as matters of Torah. What is the reason? “Your palate is like fine wine” (Song of Songs 7:10).¹²⁹The palate represents that which is stated by the Sages, while wine represents what emerges directly from the grape, a metaphor for the written Torah. The scholars [said] in the name of Rabbi Yoḥanan: The words of the scribes are dearer than matters of Torah, as it is stated: “Your palate is like fine wine.”
One who says there are no phylacteries, which contradicts matters of Torah, is exempt;¹³⁰One who denies the existence of the commandment to don phylacteries does not assume the status of a rebellious elder, as one who disputes matters written explicitly in the Torah is not considered an elder. [one who says they have] five compartments, which adds to the words of the scribes, is liable.¹³¹The head phylactery has four compartments, each with a small scroll upon which a passage from the Torah is written. This is part of the Oral Law given to Moses at Sinai. An elder who claims there are five compartments would attain the status of a rebellious elder, who is executed (see Deuteronomy 17:12). Thus, one who contravenes matters transmitted by the Sages is punished more severely than one who contradicts the Written Torah.
Rabbi Abba bar Kahana in the name of Rabbi Yehuda ben Pazi derived it from this:¹³²From the following statement, which is from the Mishna (Berakhot 1:3). Rabbi Tarfon said: I was [once] coming on the road and I stopped and reclined to recite Shema in accordance with the statement of Beit Shammai. I endangered myself due to armed robbers.¹³³Rabbi Tarfon endangered himself by stopping his travels in order to recite Shema in a reclining position, as required by Beit Shammai for the evening Shema. The Sages responded to him that he deserved to lose his life for violating the opinion of Beit Hillel, which does not require reclining for Shema. You see, had he refrained from reciting it, he would have merely violated a positive command. Now that he recited it, he is liable to lose his life; that is: The words of the scribes are dearer than matters of Torah.
Rabbi Ḥanina son of Rabbi Ada said in the name of Rabbi Tanḥum bar Aḥa: They are more stringent than matters of Torah and prophecy; it is written: “Do not preach, they preach” (Micah 2:6). This is analogous to a king who sent his officials to the provinces. Regarding one, he wrote: If he shows you my seal and my insignia, trust him, but if not, do not trust him. Regarding the other one, he wrote: Even if he does not show you my seal and my insignia, trust him. So too, regarding prophecy, it is written: “If a prophet will arise in your midst” (Deuteronomy 13:2).¹³⁴It is only when the truth of his prophecy is proven that the people are commanded to obey him. Regarding words of the scribes, it is written: “On the basis of the Torah that they will instruct you” (Deuteronomy 17:11); “that the Torah will instruct you” is not written here, but rather, “that they will instruct you.” “And the judgment that they will say to you, [you shall do;] you shall not deviate from the matter that they will tell you, right or left” (Deuteronomy 17:11) – [if they tell you that] the right is the right and the left is the left, heed them; and even if they tell you that the right is the left and that the left is the right.

2) If so, I might think that just as a spring cleanses through running waters, so a mikvah cleanses through running waters; it is, therefore, written "But a spring" — a spring cleanses through running waters, but a mikvah, through standing ones.