How can a minhag become binding?

Question

I have asked this question to a few of my Rabbeim and none have provided me with an answer.

How can something that isn't an issur d'oraysa or an issur d'rabanan, become assur?

It is extremely common in Jewish Law, that when a minhag becomes widely accepted, it has the full force of "being assur". I would like to know exactly how something that neither Hashem nor any Sanhedrin prohibited, can now become assur?

In order to help answer my question, here are a few examples that the Mishna Berura or Shulchan Aruch says are assur, that started as minhagim:

• Fasting (besides Yom Kippur and Tisha B'AV) during non-Shmad times.
• Most of the three weeks/ 9 days halachos.
• Not passing things to your wife when she is a niddah.

There are many more but I want to stay on topic. The bottom line is: How can a minhag become codified into halacha?

Answer 1

Tur, Choshen Mishpat 368 quotes a responsum of R. Sherira Gaon:

חייב כל אדם לבלתי שנות מנהגם דאמרינן מנלן דמנהגא מילתא היא שנאמר אל תסיג גבול רעך אשר גבלו ראשונים

Every person is obligated not to depart from their custom, as we say, "From where do we know that a custom is significant? As the verse (Devarim 19:14) states, 'You shall not move your countryman’s landmarks, set up by previous generations.'"

Torah Temimah, Devarim 19:14, footnote 32 quotes this comment and writes:

והלשון דאמרינן משמע דהוא מלשון חז"ל מאיזו ברייתא אבל לא מצאתיה

The expression 'as we say' implies that this quote is from the words of our sages in some beraita, but I have not found it.

A similar idea is quoted by Yalkut Shimoni in Remez 960 to Mishlei 28:22:

אל תסג גבול עולם. א"ר שמעון בן יוחאי אם ראית מנהג שעשו אבותינו אל תשנה אותו

"Do not remove the ancient boundary stone". Said R. Shimon ben Yochai, "If you saw a custom that our fathers did, do not change it."

We see from these sources that by the times of the Ge'onim / Rishonim there was a tradition that Chazal enjoined us to keep our customs, basing themselves on the similar verses of Devarim 19:14 and Mishlei 28:22.

Answer 2

There is a clal among the clalei haTalmud compiled by Shmuel Hanagid that states:

"We don't learn from the Talmud when there is a minhag by its side". This means that we don't derive laws from the Talmudic literature when there is a prevailing custom in the community. The source for this rule is based on the pasuk "al titosh Torat imecha" in Mishlei 1:8 (see Gem. Pesachim 50b also, for source regarding the ma'aseh with R' Yochanan and the people of Baishan). Therefore, a custom accepted by the majority of a community, becomes binding on the whole, even if circumstances may cause us to re-examine the minhag (ibid).

However, there are guidelines for this rule. The minhag must be based on the ruling of a Chacham or enacted by the community on their own (and doesn't entail or cause a forbidden act) and is not forbidden outright by the Torah or Talmud. Any minhag that includes a forbidden act, or causes them, may be abandoned if enacted by the community without the ruling of a Chacham. The strength behind the rule of the minhag is to maintain the derech of Tradition and is in accordance with the spirit of halacha l'Moshe m'sinai.