"He shall give him forty lashes; he may not exceed"Deuteronomy 25:2-3.

A judge may not sentence a person (found guilty of an offense that calls for flogging) to be flogged even one lash more than his body can physically bear.

A person who is sentenced to flogging is given 39 lashes. However, no person is flogged until [a doctor] estimates how many lashes the defendant can bear—taking in consideration his age, constitution and physique. If the defendant cannot bear the full 39 lashes, he is given as many as he is capable of receiving—not less, however, than three lashes. [In the event that he is only capable of bearing less than three lashes, he is not flogged at all.]

Included in this prohibition is adding even one lash on the 39 requisite lashes [even for one capable of bearing it].

Also included in this mitzvah is the prohibition against striking a fellow Jew. If we are admonished not to strike a sinner [more than he deserves], how much more so the innocent!

It is even forbidden to raise one's hand against a fellow, threatening to strike, even if one has no intention of actually hitting. As our Sages say, "One who [just] raises his hands on his fellow is called evil."