Riyad as-Salihin

Abu Hurairah (May Allah be pleased with him) reported: The Messenger of Allah (ﷺ) said, "It is not permissible for a woman who believes in Allah and the Last Day to make a journey of one day and night unless she is accompanied by a Mahram (husband or any other relative to whom she is prohibited to marry)."

[Al-Bukhari and Muslim].