Sahih Bukhari
Shortening the Prayers (At-Taqseer)
Volume: 002 / Book Number: 020 / Hadith Number: 192
Narrated Ibn 'Umar:
The Prophet said, "A woman should not travel for more than three days except with a Dhi-Mahram (i.e. a male with whom she cannot marry at all, e.g. her brother, father, grandfather, etc.) or her own husband.)"