Beitzah 15A

אלא בקשין, וכי הא דאמר רב הונא בריה דרב יהושע: האי נמטא גמדא דנרש ⁠— שריא.

Rather, it is referring to hard clothes, upon which it is permitted to sit even if they are a mixture of wool and linen. And this is in accordance with the opinion that Rav Huna, son of Rav Yehoshua, said: With regard to this hard felt [namta] material produced in the town of Neresh, it is permitted to sit or recline on it, and one need not be concerned about the fact that it is a mixture of wool and linen.

אמר רב פפא: ערדלין ⁠— אין בהן משום כלאים. אמר רבא: הני צררי דפשיטי ⁠— אין בהם משום כלאים, דבזרני ⁠— יש בהן משום כלאים. רב אשי אמר: אחד זה ואחד זה אין בהן משום כלאים, לפי שאין דרך חמום בכך.

Rav Pappa said: With regard to felt socks [ardalayin], there is no prohibition of diverse kinds regarding them, as they are hard. Rava said: These bundles for coins, comprised of hard fabric or felt, there is no prohibition of diverse kinds with regard to them. However, with regard to pouches for holding seeds, there is a prohibition of diverse kinds with regard to them, as they are larger and softer than both felt socks and hard bundles for coins. Rav Ashi said: Both this and that have no prohibition of diverse kinds with regard to them because using these items is not the usual manner of keeping warm. Even if these objects are placed close to one’s skin, this is not the usual way of wearing clothes and warming oneself, and therefore they are permitted.

אבל לא סנדל המסומר. סנדל המסומר מאי טעמא לא? משום מעשה שהיה.

§ The Mishnah taught: However, one may not send a spiked sandal on a Festival. The Talmud asks: A spiked sandal, what is the reason that it may not be worn? The Talmud answers: It is due to an incident that occurred. A great tragedy resulted when people wore spiked sandals on Shabbat, which led the rabbis to decree that these sandals may not be worn on a Shabbat or Festival.