The Kitáb-i-Aqdas - The Most Holy Book
Index term: length, under Hair
k44.
Shave not your heads; God hath adorned them with hair, and in this there are signs from the Lord of creation to those who reflect upon the requirements of nature. He, verily, is the God of strength and wisdom. Notwithstanding, it is not seemly to let the hair pass beyond the limit of the ears. Thus hath it been decreed by Him Who is the Lord of all worlds.
n69.
it is not seemly to let the hair pass beyond the limit of the ears
Shoghi Effendi has made clear that, unlike the prohibition on shaving the head, this law forbidding the growing of the hair beyond the lobe of the ear pertains only to men. The application of this law will require clarification by the Universal House of Justice.