(יז) כָּל מַחֲלֹקֶת שֶׁהִיא לְשֵׁם שָׁמַיִם, סוֹפָהּ לְהִתְקַיֵּם. וְשֶׁאֵינָהּ לְשֵׁם שָׁמַיִם, אֵין סוֹפָהּ לְהִתְקַיֵּם. אֵיזוֹ הִיא מַחֲלֹקֶת שֶׁהִיא לְשֵׁם שָׁמַיִם, זוֹ מַחֲלֹקֶת הִלֵּל וְשַׁמַּאי. וְשֶׁאֵינָהּ לְשֵׁם שָׁמַיִם, זוֹ מַחֲלֹקֶת קֹרַח וְכָל עֲדָתוֹ:
(17) Every dispute that is for the sake of Heaven, will in the end endure; But one that is not for the sake of Heaven, will not endure. Which is the controversy that is for the sake of Heaven? Such was the controversy of Hillel and Shammai. And which is the controversy that is not for the sake of Heaven? Such was the controversy of Korah and all his congregation.
(1) Now Korah, son of Izhar son of Kohath son of Levi, betook himself, along with Dathan and Abiram sons of Eliab, and On son of Peleth—descendants of Reuben— (2) to rise up against Moses, together with two hundred and fifty Israelites, chieftains of the community, chosen in the assembly, men of repute. (3) They combined against Moses and Aaron and said to them, “You have gone too far! For all the community are holy, all of them, and the LORD is in their midst. Why then do you raise yourselves above the LORD’s congregation?” (4) When Moses heard this, he fell on his face. (5) Then he spoke to Korah and all his company, saying, “Come morning, the LORD will make known who is His and who is holy, and will grant him access to Himself; He will grant access to the one He has chosen. (6) Do this: You, Korah and all your band, take fire pans, (7) and tomorrow put fire in them and lay incense on them before the LORD. Then the man whom the LORD chooses, he shall be the holy one. You have gone too far, sons of Levi!” (8) Moses said further to Korah, “Hear me, sons of Levi. (9) Is it not enough for you that the God of Israel has set you apart from the community of Israel and given you access to Him, to perform the duties of the LORD’s Tabernacle and to minister to the community and serve them? (10) Now that He has advanced you and all your fellow Levites with you, do you seek the priesthood too? (11) Truly, it is against the LORD that you and all your company have banded together. For who is Aaron that you should rail against him?”
אלו הימים שאירעו בהם צרו' לאבותינו וראוי להתענו' בהם ואע"פ שמקצתם בראש חודש יש מי שאומר שיתענו בו (וטוב שלא להשלים בראש חודש):
באחד בניסן מתו בני אהרן בעשרה בו מתה מרים ונסתלק הבאר בכ"ו בו מת יהושע בן נון ... בז' באדר מת משה רבינו ע"ה בט' בו נחלקו בית שמאי ובית הלל:
2. On the first on Nisan, the sons of Aaron died. On the tenth of it Miriam died and her well disappeared. On the twenty-sixth of it Joshua ben Nun died...On the seventh of Adar Moses, our Teacher, may peace be upon him, died. On the ninth of it Beit Hillel and Beit Shammai divided.
לבוש או"ח ס' תקפ (ר' מרדכי יפה, ,פולין)
בט' בו אדר נחלקו בית שמאי ובית הלל זה על זה, והואיל ונפלה מחלוקת בין חכמי ישראל אע"פ שמחלקותם לשם שמים היא, מ"מ נעשית התורה ח"ו (חס וחלילה) כשתי תורות (בבלי סנהדרין פח ע"ב) שזה אוסר וזה מתיר וזה מטמא וזה מטהר ואין יודעין שום דין על בוריו, הרי הוא כיום צרה ומתענין בו.
Levush, Orach Chayim, 580 (R. Mordechai Yaffe,1530-1612, Poland)
On the ninth [of Adar], Beit Shammai and Beit Hillel disagreed with one another, and since a machloket [disagreement] fell between the scholars of Israel, even though their disagreement was for the sake of heaven, nevertheless the Torah had become, G-d forbid, as if it was two Torahs, this one forbids and this one permits, this one declares a matter impure, and the other declares it pure, and no law is known completely. Behold this is like a tragic day and we fast on it.

