Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him): "It is not lawful for a woman who believes in Allah and the Last Day to travel a journey of a day and a night without a mahram."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5409