Sahih Bukhari

Narrated Ibn `Abbas:That he heard the Prophet (ﷺ) saying, ""It is not permissible for a man to be alone with a woman, and no lady should travel except with a Mahram (i.e. her husband or a person whom she cannot marry in any case for ever; e.g. her father, brother, etc.)."" Then a man got up and said, ""O Allah's Messenger (ﷺ)! I have enlisted in the army for such-and-such Ghazwa and my wife is proceeding for Hajj."" Allah's Messenger (ﷺ) said, ""Go, and perform the Hajj with your wife