Shabbat 75A

שכן יריעה שנפל בה דרנא, קורעין בה ותופרין אותה.

As, when a curtain had a worm which made a tear in it, they would tear the curtain further to lengthen the tear, and that enabled them to then sew it in a manner that obscured the tear.

אמר רב זוטרא בר טוביה אמר רב: המותח חוט של תפירה בשבת ⁠— חייב חטאת, והלומד דבר אחד מן המגוש ⁠— חייב מיתה, והיודע לחשב תקופות ומזלות ואינו מחשב ⁠— אסור לספר הימנו.

Rav Zutra bar Toviya said that Rav said: One who tightens the thread of a stitch on Shabbat is liable to bring a sin-offering. If two parts of a garment that were sewn together begin to separate, and one pulls the thread to reattach them, it is tantamount to having sewn them. The Talmud cites additional halakhot cited by Rav Zutra in the name of Rav. And one who learns even one matter from a magosh, a Persian priest, is liable to receive the death penalty. And one who knows how to calculate astronomical seasons and the movement of constellations, and does not do so, one may not speak with him because his actions are improper.

אמגושתא ⁠— רב ושמואל, חד אמר: חרשי, וחד אמר: גדופי. תסתיים דרב דאמר גדופי, דאמר רב זוטרא בר טוביה אמר רב: הלומד דבר אחד מן המגוש חייב מיתה. דאי סלקא דעתך חרשי: הכתיב ״לא תלמד לעשות״⁠ — אבל אתה למד להבין ולהורות ⁠— תסתיים.

The Talmud proceeds to discuss the additional halakhot cited by Rav Zutra bar Toviya. With regard to the magosh, Rav and Shmuel disagreed. One said that they are sorcerers, while the other said they are heretics. The Talmud adds: Conclude that Rav is the one who said that they are heretics, as Rav Zutra bar Toviya said that Rav said: One who learns one matter from the magosh is liable to receive the death penalty. As, if it should enter your mind that they are sorcerers, wasn’t it written: “When you come into the land which YHWH your God gives you, you should not learn to do after the abominations of those nations. There should not be found among you any one that makes his son or his daughter to pass through the fire, one that uses divination, a soothsayer, or an enchanter, or a sorcerer” (Deuteronomy 18:9–10)? And the rabbis inferred: You should not learn to do, but you may learn to understand and to teach the topic of sorcery. Apparently, merely learning about sorcery does not violate a prohibition. Only acting upon that learning is prohibited. Rav, who prohibited learning even a single matter from a magosh, must hold that they are heretics, not merely sorcerers. The Talmud states: Indeed, conclude that Rav is the one who said that they are heretics.