´It was narrated from Ibn 'Umar that :` The Prophet [SAW] saw a boy, part of whose head had been shaven and part had been left. He forbade that and said: "Shave all of it, or leave all of it."
Hadith Referenceسنن نسائي / كتاب الزينة من السنن / 5051
Hadith Gradingالألبانی:صحيح | زبیر علی زئی:صحيح مسلم
The disbelievers, when shaving their heads, would leave some hair on the head in dedication to an idol or the like, just as even today some ignorant people keep a lock of hair (bodi) in the name of a pir (saint). However, such veneration of other than Allah is forbidden (haram), therefore the Prophet (sallallahu alayhi wa sallam) prohibited it. Besides, this practice also appears inappropriate; a person looks unsightly, and it is contrary to human nature (fitrah). However, this does not mean that hair must be cut equally or to the same length from every part of the head. Rather, if more hair is trimmed near the ears so that it does not fall into them, and less is cut from the top of the head, there is no harm in this, provided that it appears proportionate to the eye.
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 5051
Narrated Abdullah ibn Umar: The Prophet ﷺ saw a boy with part of his head shaved and part left unshaven. He forbade them to do that, saying: Shave it all or leave it all.
Shaykh Umar Farooq Saeedi
Benefits and Issues: ➊ It is obligatory for Muslims to avoid imitation and emulation of the polytheists and disbelievers. ➋ The matter of children is related to their parents and guardians. ➌ It is incumbent upon them to keep Islamic culture in consideration regarding the clothing and haircuts of children. ➍ And when this matter is impermissible for children, then it is even more so impermissible for adults.
Source: Sunan Abu Dawood – Commentary by Shaykh Umar Farooq Saeedi, Page: 4195