Parashas Naso
כׇּל־יְמֵי֙ נֶ֣דֶר נִזְר֔וֹ תַּ֖עַר לֹא־יַעֲבֹ֣ר עַל־רֹאשׁ֑וֹ עַד־מְלֹ֨את הַיָּמִ֜ם אֲשֶׁר־יַזִּ֤יר לַיהֹוָה֙ קָדֹ֣שׁ יִהְיֶ֔ה גַּדֵּ֥ל פֶּ֖רַע שְׂעַ֥ר רֹאשֽׁוֹ׃
Throughout the term of their vow as nazirite, no razor shall touch their head; it shall remain consecrated until the completion of their term as nazirite of GOD, the hair of their head being left to grow untrimmed.
Torah Temimah points to TT / Nazir (multiple units) (verified).
כל ימי נדר נזרו — נזיר שכלו לו ימיו וגלח חייב.
English: “All the days of his nazirite vow” — a nazir whose days have been completed and who then shaves is liable.
Custom
Chazal extend the razor prohibition until the nazir’s concluding rites are performed.