Abu Hurayrah (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said: "It is not permissible for a woman who believes in Allah and the Last Day to travel the distance of one day and night unless she is accompanied by a Mahram." Another narration reads: "She must not travel the distance of a day except with a Mahram.”
شرح الحديث :
A woman is most likely desired and coveted. She cannot protect herself because of her weakness and deficient reason. It is therefore necessary that her husband or one of her male non-marriageable kin accompanies her to protect her honor, and preserve her dignity lest someone should harm her. Given this reason, the Mahram has to be an adult and of sound mind to achieve that purpose. The Lawgiver appealed to her belief in Allah and the Last Day; that if she wants to preserve her faith and comply with its requirements, she will not travel except with a Mahram.