הלכה על סנהדרין 116:16

That He prohibited the judge from striking the sinner with great lethal strikes. And the explanation of this is that the limit on all those who are liable for lashes is surely that he is lashed forty minus one, as it appears in the tradition; such that he not strike a person until he assesses the man that will be struck, according to his strength, his years, his condition and the form of his body. If he can withstand the entire striking of the punishment, he is struck [accordingly]. But if not, he is struck according to the measure of what he can withstand - [though] not less than three lashes, [based on] His saying, "according to his wickedness in number" (Deuteronomy 25:2). And the prohibition comes about adding - even one lash beyond the judge's assessment of what he can withstand - to his striking. And that is His saying, "according to his wickedness in number. Forty is he to strike him, he may not add" (Deuteronomy 25:2-3). And the language of the Sifrei (Sifrei Devarim 286:10) is, "If he does add, he transgresses a negative commandment. I only know of his adding to the forty. From where [do we know the same for his going beyond] each and every assessment that the court assessed? Hence we learn to say, 'he may not add, lest he add.'" And from this prohibition is [derived] the prohibition to strike any Israelite - if we are prohibited from striking the sinner, all the more so do we not strike any [other] person. And He already prohibited us from hinting as if one will strike, even if he does not strike. They said (Sanhedrin 58b), "Anyone who raises his hand against his fellow is called, an evildoer - as it is stated (Exodus 2:13), 'and he said to the evildoer, "Why would you strike your fellow."'" (See Parashat Shoftim; Mishneh Torah, The Sanhedrin and the Penalties within their Jurisdiction 16.)