Sahih Al Bukhari – Book of Shortening The Prayers (At-Taqseer)

Volumn 002, Book 020, Hadith Number 192.
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Narated By 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.)"