Hijab is an aspect of chastity and modesty of a Muslim woman. It is a duty that Allah Almighty prescribes for the favor of the Muslim woman, and she has to comply with the command of Allah, thus showing her sincere faith in Allah and fear of His punishment. Allah Almighty says (And it becometh not a believing man or a believing woman, when Allah and His Messenger have decided an affair (for them), that they should (after that) claim any say in their affair.) (Al-Ahzab 33: 36)
The eminent scholar, Dr. Nasr Farid Wasil, former Mufti of Egypt, stated that: Once a pilgrim, man or woman, performs all pillars of Hajj or `Umrah, then he has fulfilled his obligation and is thus exempted from further obligation.
So if a woman who intends to take off hijab after Hajj or `Umrah performed all the rituals required in the right manner and in the due time, then her Hajj or `Umrah are valid and she has fulfilled her obligation. However, whoever performs a prescribed act of worship sincerely for the sake of Allah, without intention to show off or take pride, has to abide by the teachings and manners of Islam, both during Hajj and `Umrah and after them. In that way, a person would attain Allah’s pleasure, his reward would increase, and his Hajj or `Umrah would be accepted by Almighty Allah. The Prophet (peace and blessings be upon him) said: “The performance of `Umrah is expiation for the sins committed between it and the previous ones. And the reward for Hajj mabrur (Pilgrimage accepted by Allah) is nothing but Paradise.” (Al-Bukhari)
Accordingly, a woman who intends to take off hijab after returning from Hajj or `Umrah must know that by so doing she has prior intention to commit a sin, and such intention is abhorrent enough to undermine the reward of Hajj or `Umrah because people are accountable for their deeds according to their intentions. So she has to refrain from such an intention and abide by the Islamic teachings, both before or after Hajj, so that Allah will be pleased with her on the Day of Judgment.
Moreover, we’d like to cite for you the following:
The woman who has just returned from Makkah and Madinah after the Hajj or `Umrah must improve her hijab and make it the same as the hijab of the committed Muslim woman so that her covering is an example and model not only for herself, but also for others.
That woman who has gone to Makkah but returns without the gift of the hijab of the committed Muslim woman and continues to wear her hijab in a wrong manner, or does not wear the hijab at all, has trampled over more than ten verses of the Qur’an, and her Hajj was nothing more than troubles to herself.
Yes, after completing the Hajj, the Muslim man and woman must be practical propagators of the faith through their Islamic etiquette, both among their household and amidst the people.
Excerpted with slight modifications from: al-islam.org
Finally, we would like to quote the Prophet (peace and blessings be upon him) who said: “This House of Allah (the Ka`bah) is the pillar of Islam; so whosoever heads for it with the intention of performing Hajj or `Umrah is under Allah’s Protection. If he should die (during this journey), he will be admitted to Paradise, and if he should return home safely, he will return with reward and gain.” (Al-Haythami, Majma` Az-Zawa’id) So, dear sister, do not lose Allah’s reward or disregard His pleasure so that you will not be a loser in this life and in the life to come.