Sanhedrin 41A

תנהו ענין למלקות.

apply it to the matter of lashes, as forewarning is required for the court to be able to administer lashes.

דבי חזקיה תנא: ״וכי יזד איש על רעהו להרגו בערמה״⁠ – שהתרו בו, ועדיין הוא מזיד.

The school of Ḥizkiyya taught a source for the requirement of forewarning from the verse concerning the court-imposed capital punishment meted out to a murderer, as it is states: “But if a man come intentionally upon his neighbor to slay him with guile” (Exodus 21:14). How do the witnesses know that he acted intentionally? It must be that they forewarned him, and still he acts intentionally.

דבי רבי ישמעאל תנא: ״המצאים אתו מקשש עצים״⁠ – שהתרו בו, ועדיין הוא מקושש.

The school of R' Yishmael taught a source for the requirement to forewarn transgressors from the verse concerning the court-imposed capital punishment meted out to the wood-gatherer on Shabbat in the wilderness, as it is stated: “And they that found him gathering sticks brought him” (Numbers 15:33). By writing “gathering” in the present tense, the verse indicates that they forewarned him, but he is still gathering.

דבי רבי תנא: ״על דבר אשר ענה״⁠ – על עסקי דיבור.

The school of R' Yehuda HaNasi taught a source for the requirement to forewarn a transgressor from the verse concerning the court-imposed capital punishment meted out to one who commits adultery with a betrothed young woman, as it is stated: “For the matter [devar] that he has humbled his neighbor’s wife” (Deuteronomy 22:24). They make a verbal analogy: For matters involving speech [dibbur], the punishment is given only if the witnesses issued a verbal forewarning.

וצריכא, דאי כתב רחמנא ״אחתו״⁠ – הוה אמינא: חייבי מלקות ⁠– אין, חייבי מיתות ⁠– לא. כתב רחמנא ״כי יזד״.

The Talmud comments: And it is necessary to have all of these sources, since if God would write the requirement of forewarning in the context of “his sister” (Leviticus 20:17) alone, I would say: Those liable to receive lashes, yes, they do require forewarning, but those liable to receive court-imposed capital punishments, whose transgressions are severe, do not require forewarning. Therefore God writes, with regard to a murderer: “If a man come intentionally.”