Makkot 16A

משום דהוי לאו שאין בו מעשה, וכל לאו שאין בו מעשה ⁠– אין לוקין עליו. ריש לקיש אומר: אינו לוקה, משום דהוי התראת ספק, וכל התראת ספק ⁠– לא שמה התראה.

because it is a prohibition that does not involve an action. He violates the oath by failing to perform an action, rather than by performing an action, and the principle is: With regard to any prohibition that does not involve an action, one is not flogged for its violation. Reish Lakish says: He is not flogged, because the forewarning in this case is an uncertain forewarning. One cannot properly forewarn him before he takes the oath, because as long as time remains in the day he can still eat the loaf at a later time and fulfill the oath; and any uncertain forewarning is not characterized as forewarning.

ותרוייהו אליבא דרבי יהודה. דתניא: ״ולא תותירו ממנו עד בקר, והנתר ממנו עד בקר וגו׳״, בא הכתוב ליתן עשה אחר לא תעשה, לומר שאין לוקין עליו, דברי רבי יהודה. רבי יוחנן דייק הכי: טעמא דבא הכתוב, הא לא בא הכתוב ⁠– לוקה, אלמא: התראת ספק שמה התראה.

The Talmud adds: And both R' Yoḥanan and Reish Lakish hold in accordance with the opinion of R' Yehuda, as it is taught in a baraita: It is stated with regard to the Paschal offering: “And you should let nothing of it remain until the morning, and that which remains of it until the morning you should burn in fire” (Exodus 12:10). The verse comes to position the positive mitzva of burning the leftover flesh after the prohibition against leaving over the flesh, to say that one is not flogged for its violation; this is the statement of R' Yehuda. R' Yoḥanan inferred this from the statement of R' Yehuda: The reason he is not flogged is that the verse comes and positions the mitzva after the prohibition; but if the verse had not come and positioned the mitzva after the prohibition, he would have been flogged. Apparently, uncertain forewarning is characterized as forewarning, as he can be forewarned not to leave over the flesh of the offering, even though he would not be flogged were he to burn it.