TT #631 · Parashas Ki Seitzei · Seventh Aliyah · Passuk 25:03
Forty times he shall strike him; he shall not add, lest he continue to strike him beyond these with a great blow, and your brother be degraded before your eyes.
Torah Temimah points to Makkot 23a.
TT lemma: פֶּן יֹסִיף לְהַכֹּתוֹ עַל אֵלֶּה מַכָּה רַבָּה.
English: ‘Lest he continue to strike him beyond these with a great blow’ – from here we learn that if he adds even one extra blow he is liable.
Derash establishing liability for excess (Bavli).
An officer who administers even one lash beyond the court-ordered number is himself liable for having violated the prohibition against adding.