Numbers 6:5

“For behold, you shall be with child and give birth to a son, and no razor shall come upon his head, for the boy shall be a Nazirite to God from the womb; and he shall begin to save Israel from the hand of the Philistines.”

So he told her all that was in his heart and said to her, “A razor has never come on my head, for I have been a Nazirite to God from my mother’s womb. If I am shaved, then my strength will leave me and I will become weak and be like every other man.”

And she made a vow and said, “O Yahweh of hosts, if You will indeed look on the affliction of Your maidservant and remember me and not forget Your maidservant, but will give Your maidservant a seed amongst men, then I will give him to Yahweh all the days of his life, and a razor shall never come on his head.”

“Also they shall not shave their heads, yet they shall not let their locks grow long; they shall only trim the hair of their heads.

And Paul, having remained many days longer, took leave of the brothers and put out to sea for Syria, and with him were Priscilla and Aquila. In Cenchreae he had his hair cut, for he was keeping a vow.