It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Noble Prophet (peace and blessings be upon him) said: "No woman should travel for three days without a mahram."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 4696
Hadith Gradingحکم دارالسلام:إسناده صحيح، خ : 1087، م : 1338