Bulugh al-Maram 718

He reported: I heard the Messenger of Allah preaching: “No man should be alone with a woman unless a close male relative who is unmarriageable to her is with her, and a woman should not travel without such a close male relative.” A man stood and said, “Messenger of Allah, my wife has set out for Hajj, and I have enlisted for such-and-such military expedition.” He said, “Go and perform Hajj with your wife.” The hadith is agreed upon by al-Bukhari and Muslim; the wording is Muslim's.

The editor grades it authentic and cites al-Bukhari 1862 and Muslim 1341. He refers readers to the first proof in his treatise, Awdah al-Bayan fi Hukm Safar al-Niswan (A Clear Explanation of the Ruling on Women's Travel).