Menachos Daf 40 מנחוס דַף 40

Regarding the obligation of סדין בציצית – a linen cloak in tzitzis, Beis Shammai exempts it, and forbids putting techeiles on it, because Beis Shammai holds that שעטנז was never permitted for tzitzis (rather, he must place four white linen threads). Beis Hillel obligates a linen cloak in ordinary tzitzis with techeiles, because שעטנז is overridden for tzitzis. Rebbe Elazar bar Tzadok said: והלא כל המטיל תכלת בירושלים אינו אלא מן המתמיהין – but anyone who places techeiles (on linen garments) in Yerushalayim simply astonishes everyone who sees it! Rebbe said that since the halachah follows Beis Hillel that שעטנז is permitted for tzitzis, why did they forbid it in Yerushalayim? לפי שאין בקיאין – Because they are not experts in halachah. The Gemara assumes this means they are unfamiliar with the special permit for שעטנז in tzitzis, but objects that we should publicize it. Rava eventually explains they were concerned that one’s garment may tear within three fingerbreadths of the corner, which he might sew with linen and leave a thread hanging as tzitzis, which is invalid because of "תעשה ולא מן העשוי" – “you shall make [tzitzis],” and not [use] that which was already made. Rav Zeira explains they were concerned for שעטנז in a כסות לילה – night garment, which does not require tzitzis, and therefore has no permit for שעטנז.

Rava presented a ruling which was also taught in Eretz Yisroel in Rebbe Zeira’s name: היא של בגד וכנפיה של עור חייבת – if [the garment] is made of cloth but the corners are made of leather, it is obligated in tzitzis. היא של עור וכנפיה של בגד פטורה – If [the garment] is made of leather and the corners are made of cloth, it is exempt from tzitzis. The reason for these halachos is עיקר בגד בעינן – we require the main part of the garment to be of a material obligated in tzitzis, not its corners. Rav Achai, on the other hand, אזיל בתר כנף – followed the corner of a garment in determining whether the garment was obligated in tzitzis. Thus, he arrived at the opposite ruling: regardless of what the body of the garment was made of, if the corners were cloth, he required tzitzis, and if the corners were leather, he did not require tzitzis.

Rav Huna said: הטיל לבעלת שלש והשלימה לארבע – if one placed [tzitzis] on a three-cornered [garment], and then completed it into a four-cornered [garment] and added tzitzis to the fourth corner, פסולה – it is invalid, [משום] תעשה ולא מן העשוי – because of the principle: “You shall make [tzitzis],” and not use that which was already made. The garment was not subject to tzitzis when the first three were placed onto it (since it lacked a fourth corner), and did not fulfill the mitzvah. Therefore, after the fourth corner is added, the first three cannot become valid after the fact. The Gemara challenges applying "תעשה ולא מן העשוי" to prematurely attached tzitzis, because Rebbe Zeira said: הטיל למוטלת – if one placed [tzitzis] on [a garment] which already had [tzitzis] placed onto it, the new tzitzis are valid, even if the original tzitzis are removed. Although these tzitzis were attached when they were not required, they are not disqualified because of "תעשה ולא מן העשוי"!? Rava suggests that since adding superfluous tzitzis violates "בל תוסיף" – do not add to a mitzvah, their attachment to the garment was not considered “making” tzitzis until the original threads are removed. Rav Pappa rejects this answer, because the superfluous tzitzis may have been attached to nullify the original threads (and does not violate בל תוסיף), and therefore is an event of “making” (invalid) tzitzis.