Prohibiting Woman from Traveling Alone

Ibn 'Abbas (May Allah be pleased with them) reported:
The Prophet ﷺ said, "No man must not be alone with a woman except in the presence of her (Mahram). No woman should travel except in company of a (Mahram)." A man said: "O Messenger of Allah! I have been enrolled for such and such expedition, and my wife left for Hajj." He ﷺ said to him, "Go and perform Hajj with your wife."

[Al-Bukhari and Muslim].

وعن ابن عباس رضي الله عنهما أنه سمع النبي ﷺ يقول: "لا يخلون رجل بامرأة إلا ومعها ذو محرم، ولا تسافر المرأة إلا مع ذي محرم" فقال له رجل: يا رسول الله إن امرأتي خرجت حاجّة، وإني اكتتبت في غزوة كذا وكذا؟ قال: "انطلق فحج مع امرأتك" ((متفق عليه)).