הלכה אויף שמות 2:13

“And he said to him that did the wrong, ‛Wherefore smitest thou thy fellow?’” (Exod. 2:13). He did not hit him, but nevertheless, with the raising of the hand he is called wicked.¹Sanhedrin 58b. Our sages said, “He who strikes his neighbor’s jaw, is regarded as if he had struck the jaw of the Divine Glory.” This means that he who strikes the jaw of an Israelite, as it is written, “And put a knife to thy throat” (Prov. 23:2), is regarded as if he had struck the Divine Glory.²Ibid. He has no remedy but to cut off his hand, as it is written, “And the high arm is broken” (Job 38:15).³Ibid. Therefore you, son of man, beware and guard your soul carefully, not to raise your hand against man or woman whether great or small, but only upon your son to reprove him.⁴Makkoth 8a.
/5/ The ordinance of Rabbi Gershom (960-1040) prohibited polygamy among Jews. And consider this in your heart: in the case of the man who marries two women and transgresses the Ordinance of Rabbi Gershom, the whole world says concerning him, “How impudent is that man, he is worthy to be banished and excommunicated,” all the more so does that person transgress who strikes his neighbor or raises his hand against another, even though the person is not yet thirteen years of age.⁶Mekilta, ed. Lauterbach, Exodus, III, 32.

It is forbidden to strike your fellow Jew, and if you do strike him you are transgressing a Divine negative command, as it is said, "If the wicked man deserves to be beaten etc., he should be given forty stripes, he shall not exceed, lest he exceed."¹Deuteronomy 25:2,3. If the Torah is concerned not to strike a wicked person more than prescribed for his evil, all the more so does this apply to the striking of the righteous. Whoever raises his hand against another to strike him, even if he does not actually strike him, is called a rasha (wicked person) as it is said, "And he said to the rasha: 'Why will you strike your neighbor?'"²Exodus 2:13. It is not said, "Why have you struck," but "Why will you strike;" although he did not actually strike him, he is called a rasha. Whoever strikes his fellow Jew has been excommunicated by the early Sages. He can not be counted as one of a quorum of ten required for the performance of sacred rituals, until Beis Din releases him from excommunication, upon his acceptance to abide by their decisions.³Ramah 420:1. Smah 420:4 says that from the Ramah we learn that even now, one who does this is automatically excommunicated, by virtue of this ancient ruling, and requires a Beis Din of three to annull it. Chasam Sofer Responsa 182 says that nowadays if we were to follow the example of the early Sages, we would not be able to deal with the majority of the population. However, if someone strikes you or a fellow Jew, and you are unable to save yourself or your fellow Jew from the attacker unless you strike him, you are permitted to strike him.

And the Sages reduced one of the forty in the understanding of the negative commandment of “do not increase.” So wrote Rambam, may his memory be blessed, (Mishneh Torah, The Sanhedrin and the Penalties within Their Jurisdiction 17:1), and it is a wonder [to say such a thing], according to [that which is written in] the Gemara. And this prohibition is a warning against hitting anyone of Israel. And if we are warned about not striking the sinner, is it not all the more so with all other people? And the Sages, may their memory be blessed, prevented us from even hinting to strike. They said (Sanhedrin 58b), “Anyone who raises his hand against his fellow to hit him is called an evildoer, as it states (Exodus 2:13), ‘and he said to the evildoer, “Why do you strike your fellow?”’”