Praise be to Allah.

The one who cuts his hair should make it all the same length. The one who cuts his hair on the sides of his head more than the middle comes under the heading of qaza’, which is forbidden. Al-Bukhaari (5921) and Muslim (2120) narrated from Ibn ‘Umar that the Messenger of Allaah (blessings and peace of Allaah be upon him) forbade qaza’. Naafi’ (one of the narrators of the hadeeth) said, explaining qaza’: Shaving part of a boy’s head and leaving part.

Al-Nasaa’i (5048) and Abu Dawood (4195) narrated from Ibn ‘Umar that the Prophet (blessings and peace of Allaah be upon him) saw a boy part of whose head had been shaved and part of it left. He told them not to do that and said: “Shave all of it or leave all of it.” Al-Albaani classed it as saheeh in Saheeh al-Nasaa’i.

That includes cutting some of the hair short and leaving some.

It says in Fataawa al-Lajnah al-Daa’imah (5/175):

What is the ruling on leaving some of the hair longer than the rest?

Answer: Abu Dawood narrated from ‘Abd-Allaah ibn ‘Umar (may Allaah be pleased with him) that the Messenger of Allaah (blessings and peace of Allaah be upon him) forbade qaza’ and said: “Shave all of it or leave all of it.”

it says in Sharh al-Iqnaa’: Qaza’ includes shaving some places on the sides of the head, or shaving the middle and leaving the sides, as most of the Christians do, or shaving the sides and leaving the middle, as many of the foolish do, or shaving the front and leaving the back. Ahmad was asked about shaving the back of the head and he said: This is the action of the Magians, and whoever imitates a people is one of them. Thus it is known that it is not permissible to leave some parts of the hair longer than others.

‘Abd-Allaah ibn Ghadyaan, ‘Abd al-Razzaaq ‘Afeefi, Ibraaheem ibn Muhammad Aal al-Shaykh. End quote.

Shaykh Ibn Jibreen (may Allaah preserve him) was asked: It has become common among women and men to cut their hair in such a way that there is an area that is 10 or 13 cm long, then they shave the hair on the sides and back of the head completely, leaving only the hair on the top of the head.

What is the ruling on that and what is the evidence? Does this come under the heading of qaza’ or not? Why?

He replied: There is no doubt that this action is wrong and is blind following of others, and it comes under the heading of qaza’. It was narrated that the Prophet (blessings and peace of Allaah be upon him) forbade qaza’; he saw a boy who had part of his head shaved and he said: “Shave all of it or leave all of it.” Moreover, this style is not beautification for either men or women, rather it is changing the creation of Allaah and spoiling people's appearance, and it is an imitation of the West in which there is no benefit, in addition to the cost involved, as it involves a lot of effort and spending money on something that is harmful, as is well known. We advise men not to adopt this western style and we advise women to stick with that which their mothers and grandmothers did, of letting their hair grow and braiding it, as this is more beautiful. And Allaah is the one whose help we seek; may Allaah send blessings and peace upon Muhammad and his family and Companions. End quote from Fataawa al-Shaykh Ibn Jibreen.

Shaykh Muhammad al-Mukhtaar al-Shanqeeti (may Allaah preserve him) said: The one who cut the hair on the sides of his head and makes the hair thick in the middle of the head is included in this, because this is imitating immoral people. Some of the scholars (may Allaah have mercy on them) referred to that. We know that some of our earlier shaykhs were very strict with regard to cutting some parts of the hair and not others, and they regarded that as coming under the heading of qaza’ and they said: Either cut all of it or shave all of it. This is the basic principle we should follow, according to the scholars. The Sunnah with regard to hair is to shave all of it or cut all of it, not to cut some of it and leave some of it, because this is an imitation of corrupt people. End quote from Sharh Zaad al-Mustaqni’.

See also question number 12701 and 10516.

And Allaah knows best.