Positive commandment · Mitzvat Aseh #92 of 613
For a Nazirite to grow his hair long
לגדל הנזיר שיערו
The verse · Bamidbar 6:5
כׇּל־יְמֵי נֶדֶר נִזְרוֹ תַּעַר לֹא־יַעֲבֹר עַל־רֹאשׁוֹ עַד־מְלֹאת הַיָּמִם אֲשֶׁר־יַזִּיר לַה׳ קָדֹשׁ יִהְיֶה גַּדֵּל פֶּרַע שְׂעַר רֹאשׁוֹ
“All the days of the vow of his separation there shall no razor come on his head: until the days are fulfilled, during which he separates himself to the Lord, he shall be holy, and shall let the locks of the hair of his head grow.”
The Rambam’s words
For a Nazirite to grow his hair long, as [Numbers 6:5] states: "He shall let the hair of his head grow without cutting it."
לגדל הנזיר שיערו, שנאמר: "גדל פרע, שיער ראשו" (במדבר ו,ה).
Why · Sefer HaChinuch #374
Growing the hair of the nazirite
“From the root of the commandment, I have already written an introduction at the beginning of the book that there being something in the world of the Holy One, blessed be He, that combines physicality and intellect — and that is Man — was something fitting and necessary for His praise, blessed be He, to come up properly from His creatures; that with this creature, there would not be lacking any possibilities, which we have in our minds to grasp, from His world, etc., as I wrote there.”
Who & when
“This commandment, that anyone who vows to be a nazirite is obligated to grow his hair, is practiced in all places and at all times by males and females.”
Where in the Mishneh Torah
Hilchot Nazariteship הלכות נזירות