Responding to Allaah’s command of hijab and warning against going against that command
Question

We are aware of hijab - covering of woman's face and hair. Some of my muslim lady friends donot cover their hair and there is an argument that if they tie their hair rather than keep it open, the degree of the sin (gunah) is smaller? Also keeping long hair open is a greater sin than keeping short hair open. Is it true? Plese help and reply.
Praise be to Allah.When Allaah, may He be exalted, and His Messenger (peace and blessings of Allaah be upon him) enjoin something, it is obligatory for the Muslim to say, “We hear and we obey,” and to hasten to carry out what has been enjoined upon him. This is what is required by faith in Allaah. 
Allaah says (interpretation of the meaning): 
“It is not for a believer, man or woman, when Allaah and His Messenger have decreed a matter that they should have any option in their decision. And whoever disobeys Allaah and His Messenger, he has indeed strayed into a plain error”
[al-Ahzaab 33:36]
Shaykh ‘Abd al-Rahmaan al-Sa’di (may Allaah have mercy on him) said: 
“It is not for a believer, man or woman” means: it is not appropriate or befitting for the one who is described as a believer to do anything other than to hasten to please Allaah and His Messenger, and to flee from incurring the wrath of Allaah and His Messenger, and to obey their commands, and to avoid that which they have prohibited. It is not appropriate for a believing man or a believing woman “when Allaah and His Messenger have decreed a matter” and enjoined it, “that they should have any option in their decision”. They do not have the choice of whether to do it or not, rather the believing man and the believing woman know that the Messenger is dearer to them then their own selves, so they should not let their own whims and desires form a barrier between them and obeying the command of Allaah and His Messenger. 
“And whoever disobeys Allaah and His Messenger, he has indeed strayed into a plain error” means, an obvious error, because he has forsaken the Straight Path which leads to the reward of Allaah, and has strayed to another path which leads to a painful torment. So Allaah mentions first the reason for not going against the command of Allaah and His Messenger, which is faith, then He mentions the deterrent for that, which is fear of going astray, which leads to punishment and humiliation. End quote. 
Tafseer al-Sa’di, p. 612. 
The Messenger (peace and blessings of Allaah be upon him) told us that the one who disobeys him is the one who does not want to enter Paradise! Al-Bukhaari (7280) narrated from Abu Hurayrah (may Allaah be pleased with him) that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “All of my ummah will enter Paradise except those who refuse.” They said: “O Messenger of Allaah, who would refuse?” He said: “Whoever obeys me will enter Paradise and whoever disobeys me has refused.” 
When the command of hijab came, the first Muslim women hastened to obey it, so much so that the women tore their clothes in order to hasten to obey this command. This is what is meant by faith. 
It was narrated that ‘Aa’ishah (may Allaah be pleased with her) said: May Allaah have mercy on the women of the early muhaajireen. When Allaah revealed the words “and to draw their veils all over Juyoobihinna (i.e. their bodies, faces, necks and bosoms)”, they tore their aprons and covered their faces with them. Narrated by al-Bukhaari in a mu’allaq report and by Abu Dawood (4102); classed as saheeh by al-Albaani in Saheeh Abi Dawood. 
Al-Haafiz Ibn Hajar said: 
Muroot, sing. murt (translated here as “aprons”) refers to the izaar or lower garment. 
Fath al-Baari, 8/490 
Our advice to those sisters is to hasten to obey the command of Allaah without any hesitation, and not to try to obey part of the command and neglect part of it. It is obligatory for a woman to cover her hair, face and all of her body, and it is not permissible for her to show any part of that in front of non-mahram men. Whoever does that is exposing herself to the threat and is lacking in faith to the extent that she is failing to respond to the command of Allaah. 
In the answer to question no. 11774 we have explained the ruling on women covering their faces, with detailed evidence. 
In the answer to question no. 6244 we have answered the question: “Why should women cover their hair?” 
In the answer to questions no. 214 and 6991 we have given a description of correct hijab. 
And Allaah knows best.