Reference for Makkot 31:3

ור"ל דייק הכי טעמא דבא הכתוב הא לא בא הכתוב לוקה אלמא לאו שאין בו מעשה לוקין עליו

And Reish Lakash as expounds thus, the reason [Rebbi Yehudah says that he isn't flogged] is because the Torah comes [with a remedial positive commandment], but if the Torah would not have came then he would be flogged. We therefore see that a lav sh'ein bo maiseh is subject to lashes.

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