Makkot 22B

ואמדו שאין יכול לקבל ארבעים ⁠– פטור. אמדוהו לקבל שמונה עשרה, ומשלקה אמדו שיכול הוא לקבל ארבעים ⁠– פטור.

and then they assessed him again and concluded that he cannot receive 40 lashes and survive, he is exempt from the additional lashes. If the doctors initially assessed concerning him that he is able to receive only 18 lashes, and once he was flogged 18 times they assessed that he is able to receive 40, he is exempt from receiving additional lashes.

גמ׳ מאי טעמא? אי כתיב ״ארבעים במספר״ הוה אמינא: ארבעים במניינא, השתא דכתיב ״במספר ארבעים״⁠ – מנין שהוא סוכם את הארבעים. אמר רבא: כמה טפשאי שאר אינשי, דקיימי מקמי ספר תורה, ולא קיימי מקמי גברא רבה, דאילו בספר תורה כתיב ״ארבעים״, ואתו רבנן בצרו חדא.

Talmud: The Talmud begins with a discussion of the number of lashes. What is the reason that the Rabbis said that he receives 40 lashes less one? If it had been written: 40 by number, I would say that it means 40 as a precise sum; now that it is written: “By number, 40,” the reference is to a sum that approaches 40. Likewise, Rava said: How foolish are the rest of the people who stand before a Torah scroll that passes before them, and yet they do not stand before a great man, when a Sage passes before them; as in a Torah scroll, 40 is written and the rabbis came and subtracted one, establishing the number of lashes as 39. Apparently, the authority of the rabbis is so great that they are able to amend an explicit Torah verse.

רבי יהודה אומר: ארבעים שלימות וכו׳. אמר רבי יצחק: מאי טעמא דרבי יהודה ⁠– דכתיב ״מה המכות האלה בין ידיך ואמר אשר הכתי בית מאהבי״. ורבנן? ההוא בתינוקות של בית רבן הוא דכתיב.

The Mishnah teaches: R' Yehuda says: He is flogged with a full 40 lashes, with the additional lash administered between his shoulders. R' Yitzḥak says: What is the reason for the opinion of R' Yehuda? It is as it is written: “And one should say to him: What are these wounds between your arms? Then he should answer: Those with which I was wounded in the house of my friends” (Zechariah 13:6). R' Yehuda understands that this verse is referring to one with wounds from lashes administered between his arms, indicating that there is one lash administered between the shoulders. And how do the Rabbis, who hold that one is flogged only 39 lashes, explain this verse? They explain that this verse is written with regard to schoolchildren struck by their teacher for laxity in their studies, and is not referring to lashes administered by the court.