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.

Bahá’u’lláh, The Kitab-i-Aqdas, p. 35

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.

Bahá’u’lláh: Aqdas: Notes, pages 197-198

The seventh Glad-Tidings The choice of clothing and the cut of the beard and its dressing are left to the discretion of men. But beware, O people, lest ye make yourselves the playthings of the ignorant.

Bahá’u’lláh, Tablets of Bahá’u’lláh, p. 23

It has become apparent from a number of questions we have received that many believers are not clear which are those laws already binding upon the Bahá’ís in the West. We therefore feel it is timely to clarify the situation, and the simplest way is to state those laws listed in the "Synopsis and Codification of the Kitab-i-Aqdas" which are not at present binding upon the friends in the western world [This includes] . . . the shaving of one's head and the growth of men's hair below the lobe of the ear.

Universal House of Justice, Laws from the Kitáb-i-Aqdas not Yet Binding, 9 June 1974