Why is it forbidden for a man to grow his hair long, even though the Prophet Muhammad, peace and blessings be upon him, used to grow his hair long, and what is the basis for its prohibition if it is permissible in Islamic law?

If growing one's hair is merely a personal desire and not an emulation of the Prophet's guidance (peace be upon him), then the matter differs and is not tied to a change in customs. The Prophet's action (peace be upon him) of letting his hair grow long is from the Sunnah of customs, not acts of worship. The majority of scholars hold that it signifies permissibility, not recommendation, and the reward for it depends on the intention, not the action itself.

The ruling of permissibility can change to disliking or prohibition due to an external factor unrelated to the act itself, such as the prohibition against imitating women or transgressors. As Ibn Abd al-Barr mentioned, long hair (jumam) became a sign of fools, and Ibn Muflih stipulated that growing hair should not lead to notoriety or a decrease in moral uprightness (murū’ah).

Therefore, the issue of letting one's hair grow long is subject to the same restrictions and regulations as other permissible acts and customs, such as the prohibition against imitating women and transgressors.