Numbers 6:5

For the entire period of his vow of separation, no razor shall touch his head. He must be holy until the time of his separation to the LORD is complete; he must let the hair of his head grow long.

All the days of the vow of his separation there shall no razor come upon his head: until the days be fulfilled, in the which he separateth himself unto the LORD, he shall be holy, and shall let the locks of the hair of his head grow.