The Prophet (may Allah's peace and blessings be upon him) would approach all of his wives and then take one ritual bath
Authentic hadithعن أنس: «أن النبي صلى الله عليه وسلم كان يطوف على نسائه بِغُسْلٍ واحد».
Anas (may Allah be pleased with him) reported: The Prophet (may Allah's peace and blessings be upon him) would approach all of his wives and then take one ritual bath.
Explanation
This Hadīth tells about the Prophet's good treatment to his wives. He would have sexual intercourse with all of them during one night so as to please them all. Meanwhile, he would have only one ritual bath, because washing is not obligatory between two incidents of sexual intercourse, whether it takes place with the same wife or another one, as indicated by the Hadīth.
Source · Narrated by Bukhari & Muslim
From the Hadith Encyclopedia (hadeethenc.com).