Sahih Muslim Book 7, Hadith Number 3100.

Chapter : Travelling of a woman with her Mahram for Hajj, etc.

Qaza’ah reported: I heard Abu Sa’id al-Khudri (Allah be pleased with him) saying: I heard four things from Allah’s Messenger (may peace be upon him) which impressed me and captivated me (and one out of these is this), that he forbade a woman to undertake journey extending over two days but with her husband, or with a Mahram; and he then narrated the rest of the Hadith.