Deuteronomy 25:2

והפילו השפט AND THE JUDGE SHALL CAUSE HIM TO FALL DOWN — This teaches that one is not lashed whilst standing or sitting but only when bending (Makkot 22b). לפניו כדי רשעתו [AND THE JUDGE SHALL … SMITE HIM] IN FRONT ACCORDING TO HIS WICKEDNESS (כדי רשעתו may mean “as much as is sufficient for his wickedness”, i.e., for one part of his wickedness, רשעתו being singular) — in front (on his chest) corresponding to one part of his wickedness, and on his back corresponding to two parts: hence they (the Rabbis) derived that one must give him (anyone sentenced to lashes) two-thirds on his back and one third on his chest (Sifrei Devarim 286:6; Makkot 22b, 23a). בְּמספר BY A NUMBER — but it (the word) is not punctuated בַּמספר by the number; this shows that it is connected with the following word, to read בְּמִסְפַּר ארבעים [“he shall smite him”] … by the number of forty, (i.e. by the number leading to forty), not, however, a full forty, but a number that leads up to the full total of forty, i.e., forty less one (Makkot 22a).

וְהָיָ֛ה אִם־בִּ֥ן הַכּ֖וֹת הָרָשָׁ֑ע וְהִפִּיל֤וֹ הַשֹּׁפֵט֙ וְהִכָּ֣הוּ לְפָנָ֔יו כְּדֵ֥י רִשְׁעָת֖וֹ בְּמִסְפָּֽר׃

And it shall be, if the wicked man be worthy to be beaten, that the judge shall cause him to lie down, and he shall be beaten in his presence, according to his fault, by a certain number.