Reference for Makkot 16:9

גמ' האב גולה ע"י הבן והאמרת יצא האב המכה את בנו דגמיר והאמרת אע"ג דגמיר מצוה קעביד בשוליא דנגרי

Gemara: "The father is exiled for his son." But didn't you say (in an earlier Mishnah) that a father who strikes his son is not exiled? But he was learned (which is why he would be exiled, since it was not a hit of a Mitzvah). But you said, even when he is learned, it actually is a Mitzvah that he is doing [so he would not be exiled]. [Here, we're talking about when the father is a carpenter and his son] is a carpenter's apprentice [so he is just teaching him a trade—and he would not be exiled].

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