Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) says that he heard the Messenger of Allah (peace and blessings be upon him) say: "No woman should travel a journey of two days and two nights without her husband or a mahram."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5408