Ketubot 5B

יניח אצבעו באזניו. והיינו דאמר רבי אלעזר: מפני מה אצבעותיו של אדם דומות ליתידות? מאי טעמא? אילימא משום דמחלקן, כל חדא וחדא למילתיה עבידא. דאמר מר: זו זרת. זו קמיצה. זו אמה. זו אצבע. זה גודל.

he should place his finger, which is shaped like a peg, into his ears. And that is what R' Elazar said: Why are the fingers of a person similar to pegs? The Talmud asks: What is the reason that R' Elazar said that fingers are like pegs? If we say that it is due to the fact that they are discrete from each other, each and every finger was designated for its own discrete, sacred matter, as the Master said: This small finger is for measuring a span, the distance between the little finger to the tip of the thumb used in measuring the breastplate of the High Priest; this next finger is used for taking a fistful of the meal-offering; this middle finger is used for measuring a cubit, the distance from the elbow to the tip of the middle finger; this one next to the thumb is the finger used to sprinkle the blood of offerings on the altar; this is the thumb, on which the blood and oil is placed in the purification ritual of a metzora.

אלא: מה טעם משופות כיתידות ⁠— שאם ישמע אדם דבר שאינו הגון, יניח אצבעותיו באזניו. תנא דבי רבי ישמעאל: מפני מה אוזן כולה קשה והאליה רכה? שאם ישמע אדם דבר שאינו הגון, יכוף אליה לתוכה. תנו רבנן: אל ישמיע אדם לאזניו דברים בטלים, מפני שהן נכוות תחלה לאיברים.

Rather, the question is: What is the reason that they are pointed like pegs? It is so that if a person hears an inappropriate matter, he will place his fingers in his ears. Similarly, a Sage of the school of R' Yishmael taught: Why is the entire ear hard and the earlobe soft? It is so that if a person hears an inappropriate matter, he will bend his earlobe into his ear to seal it. A baraita states: A person should not allow his ears to hear idle matters. Because of the fact that ears are very sensitive and are the first of the limbs burned, one should make certain not to expose them to anything problematic.