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].
App reference: Book 8, Hadith 35
Arabic/English book reference: Book 8, Hadith 990