Likutei Moharan 7:5/ליקוטי מוהר"ן ז׳:ה׳

15. This is the explanation (Bava Batra 74a).

2Rabbah bar bar Chanah recounted: This merchant said to me, “Come, I will show you Mount Sinai.” We went and saw that it was encircled by scorpions as tall as chamrei chivarta (white mules). I heard a Heavenly voice saying, “Woe to Me that I took an oath! But now that I have taken an oath, who can nullify it for Me?”

3Come, I will show you Mount Sinai — This is an allusion to advice, for [at Mount Sinai] the Jews received the six hundred thirteen counsels.

4we saw that it was encircled by scorpions — In other words, there is the advice of the Serpent, the counsel of the wicked, which makes it impossible to receive the advice, the “true seed,” of the tzaddikim.

5as tall as chamrei chivarta (white mules) — This alludes to the tzitzit, which protects against the Serpent’s advice, against immorality.

6{Rabbi Meir said: Not having the white threads on the tzitzit deserves greater punishment than not having the t’kheilet (sky-blue threads). Liken this to a king who issued instructions to two of his servants: one was told to bring him a seal made of clay, the other was told to bring him a seal made of gold. Each transgressed the king’s order and failed to bring the seal. Whose crime was the greater? Say, then, that the servant who failed to bring the seal of clay deserves the greater punishment, for clay is more readily available (Menachot 43b; Rashi, loc. cit.).} As our Sages taught: Which deserves greater punishment, the white or the blue? They explained: Liken it to someone who commanded that he be brought a gold seal—i.e., the blue threads—and a clay seal—i.e., the white threads. [Interpret this] as follows: ChaMRei refers to ChoMeR v’tit (clay and lime); chivarta is white, since nowadays this is the principal [requirement of the] tzitzit.

7I heard a Heavenly voice saying, Woe to Me that I took an oath! — Rashbam explains this [as God’s oath] concerning the exile. For through the tzitzit one can come to the advice of the tzaddikim, to truth. And with truth a person can achieve faith. And faith, brings the redemption, as explained. This is why [Rabbah bar bar Chanah] heard God regretting the exile: The tzitzit —the chamrei chivarta —caused all this.