Divrei Torah on the weekly portions
If you buy a Hebrew servant [who was sold to pay restitution for a robbery*] he shall serve for six years, but in the seventh year, he is to be set free without liability … If the servant … does not want to go free, his master must bring him to the court. Standing [the servant] next to the door or doorpost, his master shall pierce his ear with an awl and he shall then serve [his master until the Jubilee] (Shemot 21:2,5-6).
Rabbi Yochanan ben Zakai taught: "Why should the ear be pierced, rather than any other part of the body? It is because this ear heard on Mount Sinai the prohibition of stealing, and nevertheless proceeded to steal" (Mechilta, cited by Rashi) .
But did the ear commit the sin? Surely the hands stole the article and the legs walked to the scene of the crime, so why is the ear singled out?
Rabbi Zvi Pesach Frank explains that when a person hears the word of God it is appropriate to internalise the message throughout the body. This would make the whole body ready to observe God's commandments. However, if hearing the word of God did not achieve this for the body, it must be the case that he allowed God's word to go in one ear and out the other! Since it didn't make an impression on the rest of the body, it is as if God's words remained only in his ear. Hence, Rabbi Yochanan ben Zakai stated "this ear heard", i.e. only this ear heard, but not the other limbs. Therefore, he is pierced specifically in the ear.
Additionally, the Torah states that God's revelation at Mount Sinai was with "a loud, never-ending voice" (Devarim 5:19 and Rashi there). This teaches that we should listen continually to this never-ending voice. So, if a person is not careful to observe God's commandments, he demonstrates that he has left that voice behind at Mount Sinai and not taken it with him. Hence, Rabbi Yochanan ben Zakai stated "this ear heard on Mount Sinai", i.e. his ear only heard it at Mount Sinai but not subsequently. Therefore, he is pierced specifically in the ear.
Our commitment to Judaism should not be just to hear, but also to listen and internalise.
* See Shemot 22:2 and Rashi there.