Bava Metzia 88B

״עשר תעשר ואכלת״⁠ – ולא מוכר. ״תבואת זרעך״⁠ – ולא לוקח. אלא מדרבנן, וקרא אסמכתא בעלמא.

that a buyer need not tithe the produce he purchases, as derived from the verses: “You should tithe all the produce of your planting, which is brought forth in the field year by year, and you should eat before YHWH, your God” (Deuteronomy 14:22–23). They claimed that the phrases “you should tithe…and you should eat” indicate that only the one who eats the produce must tithe it, but not the one who sells it. Likewise, the phrase “the produce of your planting” teaches that the one who performs the planting must separate tithes, but not the one who buys it. Rather, the obligation of a buyer to separate tithes applies by rabbinic law, and the verse is cited as a mere support for this halakha. The residents of Beit Hino did not adhere to this rabbinic law and did not observe the halakha requiring the buyer to separate tithes.

אלא ״כנפשך״ למאי אתא? לכדתניא: ״כנפשך״, מה נפשך אם חסמת ⁠– פטור, אף פועל אם חסמת ⁠– פטור.

It was stated previously that a buyer is not required to tithe produce by Torah law. If so, the same certainly applies to a laborer. Consequently, the term kenafshekha cannot serve to teach that a laborer may eat without tithing, as claimed earlier. The Talmud asks: Rather, what does kenafshekha come to teach? The Talmud answers: It comes to teach that which is taught in another baraita: Kenafshekha can mean: Like your own person. Just as with regard to your own person, i.e., the owner, if you muzzled yourself and did not eat from your field’s produce, you are exempt from liability for the transgression of: “You should not muzzle an ox when it is treading out the grain” (Deuteronomy 25:4), so too with regard to a laborer, if you muzzled him, i.e., you did not allow him to eat, you are exempt from liability for the transgression of muzzling an ox while it is working.