Maliks Muwatta Book 54, Hadith Number 37.

Section : Travelling Alone in the case of Men and Women.

Malik related to me from Said ibn Abi Said al-Maqburi from Abu Hurayra that the Messenger of Allah, may Allah bless him and grant him peace, said, “It is not halal for a woman who believes in Allah and the Last Day to travel the distance of a day and night without a man who is her mahram.”