Malik related to me from Said ibn Abi Said al-Maqburi from AbuHurayra that the Messenger of Allah, may Allah bless him and grant himpeace, said, "It is not halal for a woman who believes in Allah andthe Last Day to travel the distance of a day and night without a manwho is her mahram."
USC-MSA web (English) reference: Book 54, Hadith 37
Arabic reference: Book 54, Hadith 1803