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What is the source for Rabbis being able to say that one is not allowed to do a mitzvah deorita?

Examples that come to mind are blowing shofar on Rosh Hashana on Shabbat, lulav on Sukkot (at least first day) on Shabbat, and more.

Ideas?

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In all the examples in your question, Tosafot points to the Gemara in Yavomat 89b which brings various examples and counter examples.

But at no point does the Gemara seem to bring a Pasuk to support the concept.

2 conclusions of that Gemara - and an answer to your question:

  • Their power comes - in monetary issues - from the ability of Beth Din to disown property from its owner.

  • They have the power of Shev V'al Ta'aaseh; rather sit and do nothing, (than actively do a Mitzva and risk doing a sin as a result of the Mitzva).

    • E.g.: Don't blow the shofar nor take a Lulav on Shabbat, as somebody may - as a result - carry it into the street, in a place without an Eruv.

All other examples - like Eliyahu bringing a Korban outside the Mikdash - are ascribed to one-time dispensations from which we cannot infer anything.

  • But from where do we know that they have the power of shev v'al ta'aseh? – soandos Oct 13 '15 at 22:59
  • As I already wrote "But at no point does the Gemara seem to bring a Pasuk to support the concept." I find it fascinating that it's taken for granted. R' Micha's answer (judaism.stackexchange.com/a/64482/501) tries to explain a reason for this. – Danny Schoemann Oct 14 '15 at 7:44
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This authortiy only exists in legislating gezeiros, ie laws legislated to protect the accidental violation of a deOraisa whether through error or habit. (And, the Taz adds, if the mitzvah will still usually be performed.) Such as not blowing shofar when Rosh haShanah is on Shabbos so as to avoid the possibility that someone caught up in finding someone to blow for them carrying in a reshus harabbim deOraisa.

This cannot be done for other forms of rabbinic legislation, such as to implement some broader deOraisa idea (the way reading the megillah implements pirsumei nisa -- the mitzvah to publicize a miracle).

If the prohibition comes first in time, such as the example I gave of carrying the shofar to get it to someone who can blow for you or teach you to do so, then the authority of rabbis to make such legislation is a variant of the concept of mitzvah hava'ah ba'aveirah -- one gets no credit for doing a mitzvah that was only possible because of a sin.

(The above was learned from R' Yonatan Sacks, then a rosh yeshiva at REITS, now of Lander College, in a shiur given at the Agudas Israel of Passaic Clifton. The shiur was on Shabbos, so I was unable to take notes and record sources.)

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