Eruvin 103B

מאי היא, דתניא: כהן שעלתה בו יבלת ⁠— חבירו חותכה לו בשיניו. בשיניו ⁠— אין, בכלי ⁠— לא. חבירו ⁠— אין, איהו ⁠— לא.

What is the source for this idea? As it was taught in a baraita: If a priest grew a wart, which temporarily disqualifies him from performing the service, his fellow priest may cut it off for him on Shabbat with his teeth. The Talmud infers: With his teeth, yes, this is permitted; but with an instrument, no, he may not do so. Likewise, for his fellow priest, yes, he may cut off his wart; but he himself, no, he may not cut off his own wart.

מני? אילימא רבנן ובמקדש, כיון דאמרי רבנן בעלמא משום שבות ⁠— הכא, מה לי הוא מה לי חבירו?

The Talmud inquires: According to whose opinion was this baraita taught? If you say it is in accordance with the opinion of the Rabbis, and the leniency is based on the principle that a rabbinic prohibition does not apply in the Temple, since the Rabbis say in general that biting off even one’s own nails or wart, and certainly those of another, is prohibited due to rabbinic decree, then in this case here, what is the difference to me whether it is the priest himself who cuts off the wart, or what is the difference to me whether it is another priest who cuts it off?

אלא לאו, רבי אליעזר דאמר בעלמא חייב חטאת, והכא אף על גב דמכשירי מצוה דוחין את השבת, כמה דאפשר לשנויי משנינן.

Rather, wasn’t it taught in accordance with the opinion of R' Eliezer, who said that in general one is liable to bring a sin-offering for biting off his own nails or wart? And here, even though he maintains that preparations for a mitzva override the prohibitions of Shabbat, and it should therefore be permitted for him to cut off his own wart with an instrument, nevertheless, as much as it is possible to alter the procedure so that it does not entail the violation of a Torah prohibition, one alters, and biting off another person’s wart is prohibited due to rabbinic decree, not Torah law.