I wanted to ask that why is Hijab not a part of the firu-i-din if it is compulsory.
Also, the hair of males can also be seen as a sign of beauty that can attract the opposite sex, so how comes they do not have to cover it.

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Hijaab is for both male and female in the holy quran but the way hijab is worn for women is different.
Modesty of women is more important than the mens hijab as women have been created beautifully. And woman’s attraction is deeply more sensitive due to the make up of woman’s physique