Negative commandment · Lo Ta’aseh #300 of 613
Not to add lashes when whipping a person liable for such punishment
שלא להוסיף במלקות המחוייב מלקות
The verse · Devarim 25:3
אַרְבָּעִים יַכֶּנּוּ לֹא יֹסִיף פֶּן־יֹסִיף לְהַכֹּתוֹ עַל־אֵלֶּה מַכָּה רַבָּה וְנִקְלָה אָחִיךָ לְעֵינֶיךָ
“Forty stripes he may give him, and not exceed: lest, if he should exceed, and beat him above these with many stripes, then thy brother shall be thus made vile before thee.”
The Rambam’s words
Not to add lashes when whipping a person liable for such punishment, as [Deuteronomy 25:3] states: "Do not add [lashes], lest by [giving him] this additional [punishment]...."
שלא להוסיף במלקות המחוייב מלקות, שנאמר "לא יוסיף: .פן יוסיף" (דברים כה,ג).
Why · Sefer HaChinuch #595
To not increase in flogging him
“The root of this commandment is revealed to all — as it is not right and it is not fitting to hit a creature, except according to his evil [in order] to discipline him at the behest of the court.”
Who & when
“And this prohibition is practiced concerning not hitting any Israelite in every place and at all times by males and females.”
Where in the Mishneh Torah
Hilchot The Sanhedrin and the Penalties within Their Jurisdiction הלכות סנהדרין והעונשין המסורין להם