Yalkut Shimoni on Torah 326:1

Ulla said: From where do we know forewarning from the Torah? As it is written, "And a man who takes his sister, the daughter of his father or the daughter of his mother, and sees her nakedness" (Leviticus 20:17) - and does the matter depend on seeing? Rather, it means until they show him the gravity of the matter [that is, warn him]. If the verse is not needed for the penalty of excision, apply it to the penalty of lashes. The school of Hezekiah taught: "And if a man acts with intent against his fellow to kill him by cunning" - that they forewarned him and he is still acting with intent. The school of Rabbi Yishmael taught: "those who found him gathering wood" (Numbers 15:33) - that they forewarned him and he is still gathering. The school of Rabbi taught: "because she did not cry out in the city" (Deuteronomy 22:24) - concerning matters of speech [her failure to protest implies she could have been warned]. And all three are necessary. For had the Merciful One written only regarding his sister, I would have said: those liable to lashes, yes, but those liable to court-imposed death, no; the Merciful One therefore wrote "and if a man acts with intent." And had the Merciful One written only "and if a man acts with intent," I would have said: these words apply to death by the sword, which is lenient, but to stoning, which is severe, say it does not apply; therefore it is necessary. Why are two cases needed among those who are stoned? According to Rabbi Shimon, to include those who are burned; according to the Sages, a matter that comes by an inference from minor to major Scripture nevertheless takes the trouble to write out explicitly.