Sanhedrin 85A

ורבי שמעון האמר: כל מלאכה שאינה צריכה לגופה ⁠– פטור עליה.

and doesn’t R' Shimon say: With regard to one who performs any prohibited labor that is not necessary for its own sake, it is labor for which he is exempt? In this case, causing the wound was inadvertent and was not his objective when performing the action.

בעו מיניה מרב ששת: בן מהו שיעשה שליח לאביו, להכותו ולקללו?

§ The rabbis raised a dilemma before Rav Sheshet: What is the halakha with regard to whether a son may be designated an agent of the court in order to flog his father or to curse him if the court sentenced him to ostracism?

אמר להו: ואחר מי התירו? אלא, כבוד שמים עדיף. הכא נמי, כבוד שמים עדיף.

Rav Sheshet said to them: And did the Torah render it permitted to flog or curse another who is not his father? The Torah prohibited such actions. Rather, the reason that an agent of the court may flog or curse the one found liable is that the honor of Heaven takes precedence over the honor due to another. Here too, a son may serve as an agent of the court because the honor of Heaven takes precedence over the honor due to his father.

מיתיבי: ומה מי שמצוה להכותו, מצוה שלא להכותו; מי שאינו מצוה להכותו, אינו דין שמצוה שלא להכותו?

The Talmud raises an objection from a baraita: And just as with regard to one who is commanded to strike him there is a mitzva not to strike him under other circumstances, with regard to one who is not commanded to strike him is it not right that there is a mitzva not to strike him?

מאי לאו אידי ואידי במקום מצוה? הא ⁠– בבנו, הא ⁠– באחר.

The Talmud explains the baraita: What, is it not that both this first clause of the statement of the baraita and that latter clause of the statement of the baraita are referring to a situation where striking another is a mitzva, e.g., where the other is liable to receive lashes. The distinction between them is that this latter clause, which is referring to one who is not commanded to strike him, is concerning a case where the agent is his son and is therefore prohibited from striking his father even in that capacity, while that first clause, which is referring to one who is commanded to strike him, is concerning a case where the agent is another individual and not his son. This indicates that it is prohibited for a son to flog his father as an agent of the court.