Sahih al-Bukhari — Fighting for the Cause of Allah (Jihaad) — Hadith No: 3006
Translation
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