Sahih Muslim | Book 7 | Hadith Number 3106

Narrated by Abu Huraira (Allah be pleased with him)
Abu Huraira (Allah be pleased with him) reported Allah's Messenger (may peace be upon him) as saying: It is not lawful for a woman believing in Allah and the Hereafter to undertake journey extending over a day and a night except when there is a Mahram with her.