In the name of Allah, the most Beneficent, the most Merciful.
Answer
Hair for a human is a source of beauty. Islam has ordered it’s followers to oil and brush their hair regularly and do not let it go messy or dirty. (Fathul Munim p.400 v.8)
The hairstyles that are forbidden is mentioned in the following hadeeth;
“The Prophet of Allah Sallallahu Alahi Wasalam forbade al-qaza. The narrator of the hadith asked Nafi what is qaza?” He replied; “Shaving part of a child’s hair and leaving some of his hair.”
Allahmah Kirmani (RA) has said that qaza means to shave the hair of a child in different places. (Fathul Munium P.402 V.8)
Every hairstyles that belongs to the disbelievers and immoral people is not permissible for a Muslim to resemble them.
The Prophet of Allah Sallallahu Alahi Wasalam has said in a hadith;
“He who imitates any group of people is one of them.” (Sunan Abi Dawud p.559 v.2)
Thus the conclusion we could derive is that haircuts such as short back and sides is the imitation of the immoral people and should be avoided.
Only Allah Knows Best
Mohammed Tosir Miah
Darul Ifta Birmingham