The scholars of the Standing Committee for Issuing Fatwas said:
It is established in sharee’ah that it is haraam for a woman to travel without a mahram, because the Prophet (peace and blessings of Allaah be upon him) said, according to the hadeeth of Ibn ‘Abbaas: “No woman should travel except with a mahram.” Narrated by Ahmad, al-Bukhaari and Muslim. And it was narrated that Ibn ‘Abbaas (may Allaah be pleased with him) said: I heard the Messenger of Allaah (peace and blessings of Allaah be upon him) say when he was delivering a khutbah: “No man should be alone with a woman unless she has a mahram present, and no woman should travel except with a mahram.” A man stood up and said: My wife has gone out for Hajj and I have enlisted for such and such a campaign. He said: “Go and do Hajj with your wife.” Narrated by Ahmad, al-Bukhaari and Muslim.
A woman is forbidden to undertake any kind of travel, unless she has a mahram with her to protect her and look after her. The mahram is her husband, or one to whom marriage is permanently forbidden because of blood ties, breastfeeding or ties through marriage, such as her father, son, brother, brother’s son (nephew), paternal uncle, maternal uncle, father in law, husband’s son, her son through breastfeeding, her brother through breastfeeding, and so on. This applies whether the woman is young or old, whether she is alone or with other women. A group of women is not sufficient in place of a mahram, because of the general meaning of the ahaadeeth, and because the reason (for her not to travel alone) is still there.
What women and their guardians have to do is fear Allaah and obey the commands of Allaah and His Messenger, and refrain from that which has been forbidden by Allaah and His Messenger, especially when it comes to preserving modesty and chastity and avoiding everything that may lead to evil and corruption. It is not permissible to allow desires for worldly gain to make them negligent with regard to this matter.
Hence it is not permissible for a woman to travel to perform the obligatory Hajj without a mahram, and those who are in charge of Hajj trips should not allow that, so as to avoid the sin of falling into something that the Prophet (peace and blessings of Allaah be upon him) forbade, and so as to close the door to evil and corruption. Allaah, may He be glorified and exalted, says (interpretation of the meaning):
“And Hajj (pilgrimage to Makkah) to the House (Ka‘bah) is a duty that mankind owes to Allaah, those who can afford the expenses (for one’s conveyance, provision and residence)”
[Aal ‘Imraan 3:97]
One of the conditions of a woman being able to do Hajj is that she should have a mahram who agrees to make himself available to travel with her, and Allaah does not burden any soul beyond its scope.
Fataawa al-Lajnah al-Daa’imah (17/334-336)