Ibn ‘Umar (May Allah be pleased with them) said: The Messenger of Allah (PBUH) forbade shaving a part of the head. [Al-Bukhari and Muslim]
Ibn ‘Umar (May Allah be pleased with them) said: The Messenger of Allah (PBUH) saw a boy, some portion of whose head was shaved and some of it was left out. He prohibited them from that and said, “Shave the whole of it or leave the whole of it.” [Muslim]
Commentary
Resemblance with the Jews or Christians in any mode is prohibited in Islam. Like in a hadith the dying of hair in black color is prohibited. The ahadith above cite the order of Prophet (PBUH) on shaving head. He (PBUH) forbade Muslims to shave a part of the head. If any one wants to shave the head then they should shave the whole of it or leave the whole of it. The reason behind this is that shaving a part of head was then in vogue among the Jews and Christian priests and ascetics. This style was also popular with the group which was given to mischief and disobedience. However it’s permissible in exceptional circumstances. In any case either, one should shave the hair completely or keep them in such a style that there is no resemblance with women or the disbelievers.
Islamic Encyclopedia 