Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.022A.

Abu Hurairah reported the Prophet, upon whom be peace, as saying, “Whoever has hair should honour it.” (Related by Abu Dawud.)

Said ‘Ata ibn Yasar, “A man came to the Prophet with unkempt hair and an untidy beard. The Prophet pointed to him, as if ordering him to straighten his hair and beard. He did so and returned. Thereupon the Prophet observed, ‘Is that not better than one of you coming with his hair unkempt, as if he were a devil?”‘ (Related by Malik.)

Abu Qatadah related that he had a great amount of hair. He asked the Prophet, “O Messenger of Allah, I have lots of hair. Should I comb it?” He answered, “Yes… and honour it.” Abu Qatadah used to oil it twice a day due to the Prophet’s words, “… and honour it.”

Cutting one’s hair off is permissible, and so is letting it grow if one honours it. Ibn ‘Umar narrated that the Prophet, upon whom be peace, said, “Shave it all or leave it all.” (Related by Ahmad, Muslim, Abu Dawud and an-Nasa’i). To shave part of it and leave part of it is greatly disliked. Nafa’ related from Ibn ‘Umar that the Messenger of Allah prohibited qiza’. Nafa’ asked, “What is qiza’ ?” He said, “It is to shave off part of the hair of a youth and to leave part.” (AlBukhari and Muslim.)