Riyad as-Salihin: The Book of Etiquette of Traveling – Hadith No. 311 — Ibn ‘Abbas (May Allah be pleased with them) reported: The Prophet (ﷺ) said, “No… (Hadith No. 311)

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.".

Explanation:

Reference: Riyad as-Salihin, The Book of Etiquette of Traveling, Hadith No. 311