The Gemora quotes a Braisa of Rebbi Nasan that the scriptural source for specifying the number 39 for the Avos Melachos is from the
Gematria of the word 'ayleh'. Why was this Braisa ignored by the Amoraim in Shabbos 49b who were searching for such an allusion?Dov Lew
Excellent question. (Sorry for the delay in responding.) Indeed, from the Yerushalmi (Shabbos 7:2) it seems that Rebbi's words are an additional source for the 39 Melachos, and the Amora'im mentioned on 49b had a tradition that the source could be found in various other verses.
However, in truth Rebbi's Derashah seems very perplexing. How does the Gemara know not only to make a Derashah from the word "Eleh" (which in Gematria is 36), but to add 2 from "Devarim" and another 1 from the "Heh" of "ha'Devarim?" Why does Rebbi see the need to add such numbers to the Gematria? This is especially difficult to understand in light of the Yerushalmi mentioned above, which interchanges the "Heh" into a "Ches" to make its Derashah of 39 from "Eleh!"
Perhaps, then, this is the intention of the Amora'im in our Sugya. Rebbi certainly relied on the calculation of the number of labors in the Mishkan or the number of "Melachos" in the Torah. Only as an Asmachta did he make a Derashah from "Elah."
Be well, Mordecai