Sayyiduna Abu Hurayrah (may Allah be pleased with him) says that the Prophet (peace and blessings be upon him) said: "No woman should travel (the distance of) a barid without her mahram."
Note: The narrations that mention one, two or three days are sahih (authentic).
The Prophet (peace and blessings be upon him) was asked whether a woman may travel for three days without a mahram, and he (peace and blessings be upon him) said: "No." He (peace and blessings be upon him) was asked about a journey of two days, and he (peace and blessings be upon him) said: "No." And he was asked about one day, and he (peace and blessings be upon him) said: "No." Each (narrator) reported what he remembered, and these numbers do not set any limit for a journey.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5412