The following question was asked of a marja`:
Which of the following two actions for the man is the sunnah: lengthening the hair on the head or cutting it short?
The marja,` Imam `Abdur-Rahman Ash-Shami, may Allah preserve him, answered with what follows:
It is authentically narrated from the Prophet, peace and blessings of Allah be upon him, that he either kept his hair long or cut it so that it was all one length. Thus to lengthen it is permitted but to shorten it is also permitted.
What is impermissible is doing either of these actions in order to imitate rebellious sinners in their conduct or lifestyle.
Taken from the website of the maraji`