All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon Prophet Mohammad and upon all his family and companions.
Islam encourages modesty and commands every adult discerning Muslim woman to wear Hijab in the presence of non-Mahram men (a person with whom marriage is generally permissible). This is because Allah the Almighty says {What means}: "O Prophet! Tell thy wives and daughters, and the believing women, that they should cast their outer garments over their persons (when abroad): that is most convenient, that they should be known (as such) and not molested. And God is Oft- Forgiving, Most Merciful." {Al-Ahzab, 59}. He also says {What means}: " And say to the believing women that they should lower their gaze and guard their modesty; that they should not display their beauty and ornaments except what (must ordinarily) appear thereof; that they should draw their veils over their bosoms and not display their beauty except to their husbands, their fathers, their husband’s fathers, their sons, their husbands’ sons, their brothers or their brothers’ sons, or their sisters’ sons, or their women, or the slaves whom their right hands possess, or male servants free of physical needs, or small children who have no sense of the shame of sex; and that they should not strike their feet in order to draw attention to their hidden ornaments. And O ye Believers! turn ye all together towards God, that ye may attain Bliss." {An-Nur, 31}.
According to scholarly consensus, a woman must cover her head since it is part of her ornament, which she is commanded to cover and it is included in the meaning of Awrah (The ‘awrah of a woman in front of her mahrams is her entire body except that which usually appears such as the face and hair, neck, forearms and feet. The ‘awrah of a woman with another woman is the area between the navel and the knee). It is stated in the Shafie book {Moghni Al-Mohtaj, Vol.4: p.209}: "It is forbidden for the adult discerning male, even the elderly and sexually incompetent and the effeminate to look at the Awrah of the girl that men see as a full-fledged female, even if she hasn`t reached the age of puberty, and this is according to scholarly consensus. The evidence on this is that Allah the Almighty says {What means}: "Say to the believing men that they should lower their gaze and guard their modesty" {An-Nur, 30}. Awra, here, is woman`s entire body except the face and hands."
It is an obligation upon every Muslim woman to be keen on covering all her hair since she isn`t allowed to uncover any part of it. Rather, she must tighten the Hijab on her forehead to prevent uncovering any portion of her hair. Al-Tabari {May Allah have mercy on him} interpreted the verse: "O Prophet! Tell thy wives and daughters, and the believing women, that they should cast their outer garments over their persons (when abroad)" {Al-Ahzab, 59} as follows: "They are commanded to tighten their Hijab on their foreheads." {Jami` Al-Bayan, Vol.20: p.325}.
In conclusion, it isn`t allowed for a Muslim woman to uncover any portion of her hair as this leads to grave evils and renders her prayer unacceptable in case any of her hair came out because this is part of her Awrah, which must be covered during prayer. We also advise women to fear Allah and abide by the Islamic dress code. And Allah the Almighty knows best.