Nedarim 4A

רב אחא בר יעקב אמר: כגון דנדר והוא בבית הקברות.

Rav Aḥa bar Ya’akov said: The prohibition against delaying applies to a nazirite in a case where one vowed to become a nazirite while he was in a cemetery. Since it is prohibited for a nazirite to be in a cemetery, his term of naziriteship does not come into effect until he leaves the cemetery and ritually purifies himself. If he delays this process, he prevents the vow from taking effect and thereby transgresses the prohibition against delaying.

הניחא למאן דאמר לא חיילא עליה נזירות מאלתר. אלא למאן דאמר מאלתר חיילא עליה, מי איכא ״בל תאחר״? ועוד: האמר מר בר רב אשי: חיילא נזירות עליה מאלתר, וכי פליגי ⁠— לענין מלקות פליגי!

The Talmud asks: This works out well according to the one who says that if one takes a vow of naziriteship while in a cemetery the naziriteship does not apply to him immediately. However, according to the one who said that the naziriteship applies to him immediately, is there a violation of the prohibition: You should not delay? And furthermore, didn’t Mar bar Rav Ashi say that all agree that the naziriteship applies to him immediately and he need not repeat his vow of naziriteship when he leaves, and when they disagree it is only with regard to the matter of flogging, and it’s only about this that they disagree, i.e., whether the nazirite is flogged if he drinks wine or otherwise violates the prohibitions of a nazirite.

אפילו הכי קם ליה ב״בל תאחר״, משום דקא מאחר נזירות דטהרה. אמר רב אשי: הואיל וכן, נזיר שטימא עצמו במזיד ⁠— עובר משום ״בל תאחר״ דנזירות טהרה.

The Talmud answers: Even so, he is liable for violating the prohibition: You should not delay, because by remaining in the cemetery he delays a naziriteship of ritual purity. Although the halakhot of naziriteship take effect upon him immediately, he is still obligated to fulfill a term of naziriteship in a state of ritual purity, and if he delays doing so, he violates the prohibition against delaying. Rav Ashi said: Since this is so, a nazirite who intentionally renders himself ritually impure violates the prohibition: You should not delay, with regard to a naziriteship of ritual purity, as he thereby prevents himself from completing his naziriteship in a state of ritual purity on time.